Methods of forming solar cells with fired multilayer film stacks

ABSTRACT

A method of forming a fired multilayer stack are described. The method involves the steps of a) applying a wet metal particle layer on at least a portion of a surface of a substrate, b) drying the wet metal particle layer to form a dried metal particle layer, c) applying a wet intercalation layer directly on at least a portion of the dried metal particle layer to form a multilayer stack, d) drying the multilayer stack, and e) co-firing the multilayer stack to form the fired multilayer stack. The intercalating layer may include one or more of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles. The wet metal particle layer may include aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel or combinations thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application62/259,636, filed Nov. 24, 2015, filed Apr. 5, 2016, U.S. ProvisionalPatent Application 62/318,566, filed Apr. 5, 2016, U.S. ProvisionalPatent Application 62/371,236, filed Aug. 5, 2016, and U.S. ProvisionalPatent Application 62/423,020, filed Nov. 16, 2016 all of which areincorporated by reference herein.

STATEMENT OF GOVERNMENT SUPPORT

This invention has been made with Government support under contractnumber IIP-1430721 awarded by the NSF. The Government may have certainrights in the invention.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to intercalation pastes that contain preciousmetal particles, intercalating particles and organic vehicle.

Intercalation pastes may be used to improve the power conversionefficiency of solar cells. Silver based intercalation pastes are printedon aluminum layers that have moderate peel strength after firing andsubsequent soldering to a tabbing ribbon. Such pastes may be especiallywell-suited for use in silicon based solar cells that use aluminumback-surface fields (BSF). Typically, between 85-92% of the rear surfacearea of the silicon wafer of commercially produced mono- andmulti-crystalline silicon solar cells is covered by an aluminum particlelayer, which forms a back-surface field and makes ohmic contact to thesilicon. The remaining 5-10% of the rear silicon surface is covered bythe silver rear tabbing layer, which does not produce a field and doesnot make ohmic contact to the silicon wafer. The rear tabbing layer isprimarily used to solder tabbing ribbons to electrically connect solarcells.

It is estimated that the power conversion efficiency of the solar cellsis reduced by 0.1% to 0.2% on an absolute basis when a silver layermakes direct contact to the silicon substrate on the rear side of asolar cell instead of contacting the aluminum particle layer on thesubstrate. Therefore, it is highly desirable to cover the entire backportion of the solar cell with an aluminum particle layer and still beable to connect solar cells together using tabbing ribbons. In the past,researchers have tried printing silver pastes directly on top of thealuminum particle layer, but during firing in air at high temperaturesthe aluminum and silver layers interdiffuse, and the resulting layersurface becomes oxidized and loses solderability. Some researchers haveattempted to change the atmospheric conditions to reduce oxidation;however, the front side silver pastes perform best in oxidizingatmospheres such as dry air, and overall solar cell efficiency isreduced after processing in inert atmospheres. Other researchers haveattempted to lower the peak firing temperature of the wafer to reduceinterdiffusion, but front side silver pastes require high peak firingtemperatures (i.e., more than 650° C.) to fire through silicon nitrideto make ohmic contact to the silicon substrate. Recently, researchershave used ultrasonic soldering of tin alloys directly on top of aluminumto create a solderable surface. This technique has achieved adequatepeel strength (i.e., 1-1.5 N/mm) but requires additional equipment anduses a large quantity of tin, which adds cost. Furthermore, usingultrasonic soldering on brittle materials such as aluminum and siliconwafer can increase wafer breakage and decrease processing yields.

There is a need to develop printable pastes that can modify the materialproperties of underlying metal particle layers during firing. Forexample, precious metal containing pastes that can be directly printedon aluminum and fired using standard solar cell processing conditionscould improve solar cell efficiency. These pastes should reduce Ag/Alinterdiffusion in order to remain solderable to the tabbing ribbon. Itis desirable for the paste to be screen-printable and act as a drop-inreplacement, which would result in no additional capital expenses andcan be immediately integrated into existing production lines.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and others will be readily appreciated by theskilled artisan from the following description of illustrativeembodiments when read in conjunction with the accompanying drawings. Thefigures are not drawn to scale. The drawings are illustrative only andare not intended to be exhaustive or to limit the invention.

FIG. 1 is a schematic cross-section drawing of multilayer stack prior tofiring, according to an embodiment of the invention.

FIG. 2 is a schematic cross-section drawing of a fired multilayer stack,according to an embodiment of the invention.

FIG. 3 is a schematic cross-section drawing of a fired multilayer stackin which the intercalation layer has phase separated.

FIG. 4 is a schematic cross-section drawing of a fired multilayer stackin which the intercalation layer has phase separated into two sublayers.

FIG. 5 is a schematic cross-section drawing of a portion of the firedmultilayer stack shown in FIG. 2, according to an embodiment of theinvention.

FIG. 6 is a scanning electron microscope (SEM) cross-section image of aco-fired multilayer stack, according to an embodiment of the invention.

FIG. 7 is a scanning electron micrograph (SEM) cross-section image of aco-fired multilayer stack that has a silver-bismuth frit layer.

FIG. 8 is a scanning electron microscope (SEM) cross-section image (inSE2 mode) of an aluminum particle layer on a silicon substrate.

FIG. 9 is a scanning electron microscope (SEM) cross-section image (inInLens mode) of an aluminum particle layer on a silicon substrate shownin FIG. 8.

FIG. 10 is a scanning electron microscope (SEM) cross-section image (inInLens mode) of a portion of a silicon solar cell that contains aco-fired multilayer stack.

FIG. 11 is a scanning electron microscope (SEM) cross-section image (inSE2 mode) of the portion of a silicon solar cell that contains aco-fired multilayer stack shown in FIG. 10.

FIG. 12 shows energy dispersive x-ray (EDX) spectra from an aluminumparticle layer and from a modified aluminum particle layer, according toan embodiment of the invention.

FIG. 13 is an EDX spectrum of the surface of a rear tabbing layer thatcontains a silver-bismuth intercalation layer, according to anembodiment of the invention.

FIG. 14 shows x-ray diffraction patterns from co-fired multilayer filmstacks on the rear tabbing layer of a silicon solar cell.

FIG. 15 is a schematic cross-section drawing of multilayer film stackthat includes a dielectric layer prior to firing, according to anembodiment of the invention.

FIG. 16 is a schematic cross-section drawing of a fired multilayer filmstack that includes a dielectric layer, according to an embodiment ofthe invention.

FIG. 17 is a plan-view optical micrograph of co-fired multilayer filmstack in which buckling has occurred.

FIG. 18 is a screen design (not drawn to scale) that can be used duringdeposition of a wet metal particle layer, according to an embodiment ofthe invention.

FIG. 19 is a schematic cross-section drawing of a dried metal particlelayer with variable thickness deposited using the screen in FIG. 18,according to an embodiment of the invention.

FIG. 20 is a schematic cross-section drawing of a modified metalparticle layer with variable thickness deposited using the screen ofFIG. 18 and then co-fired, according to an embodiment of the invention.

FIG. 21 is a plan-view optical micrograph of a co-fired multilayer stackas shown in FIG. 20.

FIG. 22 is a cross-section SEM image of a portion of a fired multilayerstack with variable thickness.

FIG. 23 is a cross-section SEM image of a portion of an aluminumparticle film on a silicon substrate with a planar thickness.

FIG. 24 is a surface topology scan of a fired multilayered stack withvariable thickness.

FIG. 25 is a surface topology scan of a fired aluminum particle layer.

FIG. 26 is a schematic drawing that shows the front (or illuminated)side of a silicon solar cell.

FIG. 27 is a schematic drawing that shows the rear side of a siliconsolar cell.

FIG. 28 is a schematic cross-section drawing of a solar cell module thatincludes a fired multilayer stack, according to an embodiment of theinvention.

FIG. 29 is a scanning electron micrograph (SEM) cross-section of theback side of a solar cell that includes a fired multilayer stack and asoldered tabbing ribbon, according to an embodiment of the invention.

FIG. 30 is a transmission line measurement plot of a conventional silverrear tabbing layer on silicon.

FIG. 31 is a transmission line measurement plot of an silver-bismuthintercalation layer on an aluminum particle layer that can be used as arear tabbing layer on silicon.

SUMMARY

A fired multilayer stack is disclosed. In one embodiment of theinvention, the stack has a substrate, a metal particle layer on at leasta portion of the substrate surface, a modified metal particle layer onat least a portion of the substrate surface, and a modifiedintercalation layer directly on at least a portion of the modified metalparticle layer. The modified intercalation layer has a solderablesurface that faces away from the substrate. The modified metal particlelayer includes the same metal particles that are in the metal particlelayer and at least one material from the modified intercalation layer.The modified intercalation layer contain a precious metal and a materialselected from the group consisting of antimony, arsenic, barium,bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt,gallium, germanium, hafnium, indium, iron, lanthanum, lead, lithium,magnesium, manganese, molybdenum, niobium, phosphorous, potassium,rhenium, selenium, silicon, sodium, strontium, sulfur, tellurium, tin,vanadium, zinc, zirconium, combinations thereof, and alloys thereof,oxides thereof, composites thereof, and other combinations thereof. Inone arrangement, the modified intercalation layer contain a preciousmetal and a material selected from the group consisting of bismuth,boron, indium, lead, silicon, tellurium, tin, vanadium, zinc,combinations thereof, and alloys thereof, oxides thereof, compositesthereof, and other combinations thereof.

In one embodiment of the invention, the modified intercalation layer hastwo phases: a precious metal phase and an intercalation phase. More than50% of the solderable surface of the modified intercalation layer maycontain the precious metal phase. The modified metal particle layer mayinclude the metal particles discussed above and at least one materialfrom the intercalation phase. The intercalation phase comprises amaterial selected from the group consisting of antimony, arsenic,barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium,cobalt, gallium, germanium, hafnium, indium, iron, lanthanum, lead,lithium, magnesium, manganese, molybdenum, niobium, phosphorous,potassium, rhenium, selenium, silicon, sodium, strontium, sulfur,tellurium, tin, vanadium, zinc, zirconium, combinations thereof, andalloys thereof, oxides thereof, composites thereof, and othercombinations thereof. The precious metal phase includes at least onematerial selected from the group consisting of gold, silver, platinum,palladium, rhodium, and alloys, composites, and other combinationsthereof.

In another embodiment of the invention, the modified intercalation layerhas two sublayers: an intercalation sublayer directly on at least aportion of the modified metal particle layer and a precious metalsublayer directly on at least a portion of the intercalation sublayer.The solderable surface of the modified intercalation layer contains theprecious metal sublayer. The modified metal particle layer may includethe metal particles discussed above and at least one material from theintercalation sublayer. Possible materials for the intercalationsublayer are the same as were described above for the intercalationphase. Possible materials for the precious metal sublayer are the sameas were described above for the precious metal phase.

In another embodiment of the invention, a fired multilayer stack has amodified aluminum particle layer as its modified metal particle layer.It has a modified intercalation layer that has two sublayers: abismuth-rich sublayer directly on the modified aluminum particle layer;and a silver-rich sublayer directly on the bismuth-rich sublayer. Thesolderable surface of the modified intercalation layer contains thesilver-rich sublayer. The modified aluminum particle contains aluminumparticles and may also contain at least one material selected from thegroup consisting of aluminum oxides, bismuth, and bismuth oxides.

In one arrangement, there is at least one dielectric layer directly onat least a portion of the substrate surface. The dielectric layerincludes at least one material selected from the group consisting ofsilicon, aluminum, germanium, hafnium, gallium, and oxides, nitrides,composites, and combinations thereof. In another arrangement, there isan aluminum oxide dielectric layer directly on at least a portion of thesubstrate surface and a silicon nitride dielectric layer directly on thealuminum oxide dielectric layer.

In one arrangement, there is a solid (e.g., eutectic) compound layerdirectly on the substrate surface. The solid compound layer includes oneor more metals selected from the group consisting of aluminum, copper,iron, nickel, molybdenum, tungsten, tantalum, titanium and one or morematerials selected from the group consisting of silicon, oxygen, carbon,germanium, gallium, arsenic, nitrogen, indium and phosphorous.

A portion of the substrate adjacent to the substrate surface may bedoped with at least one material selected from the group consisting ofaluminum, copper, iron, nickel, molybdenum, tungsten, tantalum,titanium, steel and combinations thereof.

In one embodiment of the invention, a portion of the fired multilayerstack has variable thickness. The fired multilayer stack may have anaverage peak to valley height greater than 12 μm.

At least 70 wt % of the solderable surface of the modified intercalationlayer may include a material selected from the group consisting ofsilver, gold, platinum, palladium, rhodium, and alloys, composites, andother combinations thereof.

The substrate may include at least one material selected from the groupconsisting of silicon, silicon dioxide, silicon carbide, aluminum oxide,sapphire, germanium, gallium arsenide, gallium nitride, and indiumphosphide. Alternatively, the substrate may include a material selectedfrom the group consisting of aluminum, copper, iron, nickel, titanium,steel, zinc, and alloys, composites, and other combinations thereof. Themetal particle layer may include a material selected from the groupconsisting of aluminum, copper, iron, nickel, molybdenum, tungsten,tantalum, titanium, steel and alloys, composites, and other combinationsthereof. The precious metal may include a material selected from thegroup consisting of silver, gold, platinum, palladium, rhodium, andalloys, composites, and other combinations thereof.

The metal particle layer may have a thickness between 0.5 μm and 100 μmand/or a porosity between 1 and 50%. The modified intercalation layermay have a thickness between 0.5 μm and 10 μm. The fired multilayerstack may have a contact resistance between 0 and 5 mOhm, as determinedby transmission line measurement.

There may also be a tabbing ribbon directly on at least a portion of thesolderable surface of the modified intercalation layer. In onearrangement, the peel strength between the tabbing ribbon and themodified intercalation layer is greater than 1 N/mm.

In another embodiment of the invention, a fired multilayer stack has asubstrate, a metal particle layer on at least a portion of thesubstrate, a modified metal particle layer on at least a portion of thesubstrate, and a modified intercalation layer directly on at least aportion of the modified metal particle layer. The modified intercalationlayer has two sublayers: an intercalation sublayer directly on at leasta portion of the modified metal particle layer and a precious metalsublayer directly on at least a portion of the intercalation sublayer.The modified metal particle layer includes metal particles and at leastone material from the intercalation sublayer. Possible materials for theintercalation sublayer have been described above.

In another embodiment of the invention, a fired multilayer stack has asilicon substrate, an aluminum particle layer on at least a portion ofthe substrate, a modified aluminum particle layer on at least a portionof the substrate, and a modified intercalation layer directly on themodified aluminum particle layer. The modified intercalation layer hastwo sublayers: a bismuth-rich sublayer directly on the modified aluminumparticle layer and a silver-rich sublayer directly on the bismuth-richsublayer. The modified aluminum particle layer includes at least onematerial selected from the group consisting of aluminum, aluminumoxides, bismuth, and bismuth oxides.

In one embodiment of the invention, a solar cell has a siliconsubstrate, at least one front dielectric layer directly on at least aportion of the front surface of the silicon substrate, a plurality offine grid lines on a portion of the front surface of the siliconsubstrate, at least one front busbar layer in electrical contact with atleast one of the plurality of fine grid lines, an aluminum particlelayer on at least a portion of the rear surface of the siliconsubstrate, and a rear tabbing layer on a portion of the rear surface ofthe silicon substrate. The rear tabbing layer includes a modifiedaluminum particle layer on a portion of the rear surface of the siliconsubstrate and a modified intercalation layer directly on at least aportion of the modified aluminum particle layer. The modifiedintercalation layer has a solderable surface that faces away from thesilicon substrate. The modified aluminum particle layer includesaluminum particles and at least one material from the modifiedintercalation layer. Possible materials for the modified intercalationlayer have been described above. The aluminum particle layer may have athickness between 1 μm and 50 μm and/or a porosity between 3 and 20%.The rear tabbing layer may have a thickness between 1 μm and 50 μm. Thesilicon substrate may be a monocrystalline silicon wafer with either ap-type base or an n-type base. The silicon substrate may be amulti-crystalline silicon wafer with either a p-type base or an n-typebase.

In one embodiment of the invention, the modified intercalation layerincludes two phases: a precious metal phase and an intercalation phase.More than 50% of the solderable surface may be made up of the preciousmetal phase. The modified aluminum particle layer includes aluminumparticles and at least one material from the intercalation phase.Possible materials for the intercalation phase have been describedabove. Possible materials for the precious metal phase have beendescribed above.

In another embodiment of the invention, the modified intercalation layerincludes two sublayers: an intercalation sublayer directly on at least aportion of the modified metal particle layer, and a precious metalsublayer directly on at least a portion of the intercalation sublayer.The solderable surface contains the precious metal sublayer. Themodified aluminum particle layer includes aluminum particles and atleast one material from the intercalation sublayer. Possible materialsfor the intercalation sublayer have been described above. Possiblematerials for the precious metal sublayer have been described above.

In another embodiment of the invention, the modified intercalation layercomprises two sublayers: a bismuth-rich sublayer directly on themodified aluminum particle layer, and a silver-rich sublayer directly onthe bismuth-rich sublayer. The modified aluminum particle layer furtherincludes at least one material selected from the group consisting ofaluminum oxides, bismuth, and bismuth oxides. In one arrangement,modified aluminum particle layer further includes bismuth and/or bismuthoxide and the weight ratio of bismuth to bismuth and aluminum(Bi:(Bi+A1)) is at least 20% higher in the modified aluminum particlelayer than in the aluminum particle layer. The bismuth-rich sublayer mayhave a thickness between 0.01 μm and 5 μm or between 0.25 μm and 5 μm.

In one arrangement, there is at least one rear dielectric layer directlyon at least a portion of the rear surface of the silicon substrate. Therear dielectric layer includes one or more of silicon, aluminum,germanium, hafnium, gallium, and oxides, nitrides, composites, andcombinations thereof. The rear dielectric layer may contain siliconnitride. In another arrangement, there is an aluminum oxide reardielectric layer directly on at least a portion of the rear surface ofthe silicon substrate and a silicon nitride rear dielectric layerdirectly on the aluminum oxide rear dielectric layer. In onearrangement, there is a solidified aluminum-silicon eutectic layerdirectly on the silicon substrate. In one arrangement, a portion of thesilicon substrate adjacent to the rear surface of the silicon substratefurther comprises a rear surface field and the rear surface field isdoped p-type to between 10¹⁷ and 10²⁰ atoms per cm³.

In one embodiment of the invention, a portion of the rear tabbing layerhas variable thickness and may have an average peak to valley heightgreater than 12 μm.

There may be a tabbing ribbon directly on at least a portion of thesolderable surface of the modified intercalation layer. The solderablesurface may be silver rich. The solderable surface may contain at least75 wt % silver. A tabbing ribbon soldered to a silver-rich solderablesurface may have a peel strength greater than 1 N/mm.

A portion of the modified aluminum particle layer may have variablethickness. A portion of the modified aluminum particle layer may have anaverage peak to valley height greater than 12 μm. Contact resistancebetween the rear tabbing layer and the aluminum particle layer may bebetween 0 and 5 mOhm as determined by transmission line measurement.

In another embodiment of the invention, a solar cell has a siliconsubstrate, at least one front dielectric layer directly on at least aportion of the front surface of the silicon substrate, a plurality offine grid lines on a portion of the front surface of the siliconsubstrate, at least one front busbar layer in electrical contact with atleast one of the plurality of fine grid lines, an aluminum particlelayer on at least a portion of the rear surface of the siliconsubstrate, and a rear tabbing layer on a portion of the rear surface ofthe silicon substrate. The rear tabbing layer has a solderable surface.The rear tabbing layer includes a modified aluminum particle layer on atleast a portion of the rear surface of the silicon substrate, abismuth-rich sublayer directly on at least a portion of the modifiedaluminum particle layer, and a silver-rich sublayer directly on at leasta portion of the bismuth-rich sublayer. The modified aluminum particlelayer contains aluminum particles and at least one material selectedfrom the group consisting of aluminum oxides, bismuth, and bismuthoxides.

In another embodiment of the invention, a solar cell module has a frontsheet, a front encapsulant layer on the rear surface of the front sheet,and a first silicon solar cell and a second silicon solar cell on thefront encapsulant layer. Each silicon solar cell can be any of thesilicon solar cells described herein. The solar cell module also has afirst cell interconnect that includes a first tabbing ribbon inelectrical contact with both the front busbar layer of the first siliconsolar cell, and the rear tabbing layer of the second silicon solar cell,a rear sheet, a rear encapsulant layer on the rear surface of the rearsheet. A first portion of the rear encapsulant layer is in contact withthe first silicon solar cell and the second solar cell, and a secondportion of the rear encapsulant layer is in contact with the frontencapsulant layer.

The first cell interconnect may also include a junction box in contactwith the rear sheet. The junction box may contain at least one bypassdiode. There may also be at least one busbar ribbon connecting to thefirst tabbing ribbon.

In one embodiment of the invention, a paste is disclosed. The pastecontains between 10 wt % and 70 wt % precious metal particles, at least10 wt % intercalating particles and organic vehicle. The intercalatingparticles include one or more selected from the group consisting of lowtemperature base metal particles, crystalline metal oxide particles, andglass frit particles. The weight ratio of the intercalating particles tothe precious metal particles may be at least 1:5.

The precious metal particles may include at least one material selectedfrom the group consisting of gold, silver, platinum, palladium, rhodium,and alloys, composites, and other combinations thereof. The preciousmetal particles may have a D50 between 100 nm and 50 μm and a specificsurface area between 0.4 and 7.0 m²/g. A portion of the precious metalparticles may have shapes such as a spherical shapes, flake shapes,and/or elongated shapes. The precious metal particles may have aunimodal size distribution or a multimodal size distribution. In onearrangement, the precious metal particles are silver and have a D50between 300 nm and 2.5 μm and a specific surface area between 1.0 and3.0 m²/g.

The intercalating particles may have a D50 between 100 nm and 50 μm anda specific surface area between 0.1 and 6.0 m²/g. A portion of theintercalating particles may have shapes such as a spherical shapes,flake shapes, and/or elongated shapes. The intercalating particles mayhave a unimodal size distribution or a multimodal size distribution.

The low temperature base metal particles may include a material selectedfrom the group consisting of bismuth, tin, tellurium, antimony, lead,and alloys, composites, and other combinations thereof. In onearrangement, the low temperature base metal particles contain bismuthand have a D50 between 1.5 and 4.0 μm and a specific surface areabetween 1.0 and 2.0 m²/g.

In one embodiment of the invention, at least some of the low temperaturebase metal particles have a bismuth core particle surrounded by a singleshell that includes a material selected from the group consisting ofsilver, nickel, nickel-boron, tin, tellurium, antimony, lead,molybdenum, titanium, and alloys, composites, and other combinationsthereof. In another embodiment, at least some of the low temperaturebase metal particles have a bismuth core particle surrounded by a singleshell that comprises a material selected from the group consisting ofsilicon oxides, magnesium oxides, boron oxides, and any combinationthereof.

The crystalline metal oxide particles may contain oxygen and a metalselected from the group consisting of bismuth, tin, tellurium, antimony,lead, vanadium, chromium, molybdenum, boron, manganese, cobalt, andalloys, composites and other combinations thereof.

The glass frit particles may contain a material selected from a groupconsisting of antimony, arsenic, barium, bismuth, boron, cadmium,calcium, cerium, cesium, chromium, cobalt, fluorine, gallium, germanium,hafnium, indium, iodine, iron, lanthanum, lead, lithium, magnesium,manganese, molybdenum, niobium, potassium, rhenium, selenium, silicon,sodium, strontium, tellurium, tin, vanadium, zinc, zirconium, alloysthereof, oxides thereof, composites thereof, and other combinationsthereof.

The paste may have a solids loading between 30 wt % and 80 wt %. Theintercalating particles may make up at least 15 wt % of the paste. Inone arrangement, the paste inlcudes 45 wt % Ag particles, 30 wt %bismuth particles, and 25 wt % organic vehicle. In another arrangement,the paste includes 30 wt % Ag particles, 20 wt % bismuth particles, and50 wt % organic vehicle. The paste may have a viscosity between 10,000and 200,000 cP at 25° C. at a sheer rate of 4 sec⁻¹.

In one embodiment of the invention, a co-firing method of forming afired multilayer stack, is described. The method involves the steps ofa) applying a wet metal particle layer on at least a portion of asurface of a substrate, b) drying the wet metal particle layer to form adried metal particle layer, c) applying a wet intercalation layerdirectly on at least a portion of the dried metal particle layer to forma multilayer stack, d) drying the multilayer stack, and e) co-firing themultilayer stack to form the fired multilayer stack.

In another embodiment of the invention, a sequential method of forming afired multilayer stack, is described. The method involves the steps ofa) applying a wet metal particle layer on at least a portion of asurface of a substrate, b) drying the wet metal particle layer to form adried metal particle layer, c) firing the dried metal particle layer toform a metal particle layer, d) applying a wet intercalation layerdirectly on at least a portion of the metal particle layer to form amultilayer stack, e) drying the multilayer stack, and f) firing themultilayer stack to form the fired multilayer stack.

In one arrangement, for both the co-firing method and the sequentialmethod, the wet intercalation layer has between 10 wt % and 70 wt %precious metal particles, at least 10 wt % intercalating particles, andorganic vehicle. The intercalating particles may include one or moreselected from the group consisting of low temperature base metalparticles, crystalline metal oxide particles, and glass frit particles.The wet metal particle layer may include metal particles consisting of amaterial selected from the group comprising aluminum, copper, iron,nickel, molybdenum, tungsten, tantalum, titanium, steel and alloys,composites, and other combinations thereof.

In one arrangement, for both the co-firing method and the sequentialmethod, there is an additional step before step a). The additional stepinvolves depositing at least one dielectric layer onto at least aportion of the surface of the substrate. In this arrangement, step a)involves applying the wet metal particle layer directly on at least aportion the dielectric layer.

For both the co-firing method and the sequential method, each applyingstep may involve a method selected from the group consisting of screenprinting, gravure printing, spray deposition, slot coating, 3D printingand inkjet printing. In one arrangement, step a) involves screenprinting through a patterned screen to produce a wet metal particlelayer that has variable thickness.

For both the co-firing method and the sequential method, steps b) and d)may involve drying at a temperature below 500° C. for between 1 secondand 90 minutes or at a temperature between 150° C. and 300° C. forbetween 1 second and 60 minutes. Step e) may involve rapidly heating toa temperature greater than 600° C. for between 0.5 second and 60 minutesin air or rapidly heating to a temperature greater than 700° C. forbetween 0.5 and 3 seconds in air.

In one arrangement, for both the co-firing method and the sequentialmethod, an additional step f) involves soldering a tabbing ribbon onto aportion of the fired multilayer stack.

Low temperature base metal particles, crystalline metal oxide particles,glass frit particles, and metal particle layers are described in detailabove.

In another embodiment of the invention, a method of fabricating a solarcell involves the steps of: a) providing a silicon wafer, b) applying awet aluminum particle layer on at least a portion of the back surface ofthe silicon wafer, c) drying the wet aluminum particle layer to form analuminum particle layer, d) applying a wet intercalation layer directlyon at least a portion of the aluminum particle layer to form amultilayer stack, e) drying the multilayer stack, f) applying aplurality of fine grid lines and at least one front busbar layer ontothe front surface of the silicon wafer, g) drying the plurality of finegrid lines and the at least one front busbar layer to form a structure,and h) co-firing the structure to form a silicon solar cell.

The wet intercalation layer has been described above.

In one arrangement, there is an additional step between step a) and stepb). The additional step involves depositing at least one dielectriclayer onto at least a portion of the back surface of the silicon wafer.In this arrangement, step b) involves applying the wet aluminum particlelayer directly on at least a portion the dielectric layer.

Each applying step may involve a method selected from the groupconsisting of screen printing, gravure printing, spray deposition, slotcoating, 3D printing and inkjet printing. In one arrangement, step b)involves screen printing through a patterned screen to produce a wetaluminum particle layer that has variable thickness.

For both the co-firing method and the sequential method, steps e) and g)may involve drying at a temperature below 500° C. for between 1 secondand 90 minutes or at a temperature between 150° C. and 300° C. forbetween 1 second and 60 minutes. Step h) may involve rapidly heating toa temperature greater than 600° C. for between 0.5 second and 60 minutesin air or rapidly heating to a temperature greater than 700° C. forbetween 0.5 and 3 seconds in air. Low temperature base metal particles,crystalline metal oxide particles, and glass frit particles have beendescribed in detail above.

DETAILED DESCRIPTION

The preferred embodiments are illustrated in the context of firedintercalation pastes on metal particle layers. The skilled artisan willreadily appreciate, however, that the materials and methods disclosedherein will have application in a number of contexts where making goodelectrical contact to semiconducting or conducting materials isdesirable, particularly where good adhesion, high performance, and lowcost are important.

All publications referred to herein are incorporated by reference intheir entirety for all purposes as if fully set forth herein.

Herein is disclosed the composition and use of intercalation pastescomprising precious metal particles and intercalating particles, whichcan be printed on metal particle layers to alter the properties of themetal particle layers after they have been fired into a fired multilayerstack. In one embodiment of the invention, intercalation pastes are usedto provide a solderable surface on a metal particle layer that would nothave been solderable by itself. Intercalation pastes may also be used toimprove adhesion within a fired multilayer stack or to change theinteraction of a metal particle layer with an underlying substrate.Intercalation pastes are widely applicable to many applicationsincluding transistors, light emitting diodes, and integrated circuits;however, the majority of examples disclosed below will focus onphotovoltaic cells.

Definitions and Methods

Scanning electron microscopy (SEM) and energy dispersive x-rayspectroscopy (EDX) (referred to collectively as SEM/EDX) as used hereinwere performed using a Zeiss Gemini Ultra-55 analytical field emissionscanning electron microscope, equipped with a Bruker) (Flash® 6|60detector. Details about operating conditions are described for eachanalysis. Cross-sectional SEM images of the fired multilayer stack wereprepared by ion milling. A thin epoxy layer was applied to the top ofthe fired multilayer stack and dried for at least 30 minutes. The samplewas then transferred to a JEOL IB-03010CP ion mill operating at 5 kV and120 uA for 8 hours to remove 80 microns from the sample edge. Milledsamples were stored in a nitrogen glove box prior to SEM/EDX.

The term “drying” describes a thermal treatment at or below atemperature of 500° C., or below 400° C., or below 300° C. for a timeperiod between 1 second and 90 minutes or any range subsumed therein.Pastes are typically applied to a substrate via screen printing oranother deposition method to create a “wet” layer. The wet layer may bedried to reduce or remove volatile organic species such as solvents,creating a “dried” layer.

The term “firing” describes heating at a temperature greater than 500°C., greater than 600° C., or greater than 700° C. for a time periodbetween 1 second and 60 minutes, or any range subsumed therein. The term“fired layer” describes a dried layer that has been fired.

The term “multilayer stack” is used herein to describe a substrate thathas two or more layers of different materials on it. A “fired multilayerstack” is a multilayer stack whose layers have been dried and fired.There are several ways to fire such a multilayer stack. The term“co-firing” is used to describe the treatment for multilayer stacks thatare fired only once. For example, during silicon solar cell fabrication,a layer of aluminum particle paste is first applied to a substrate anddried. Then, a rear tabbing paste layer is applied on a portion of thedried aluminum particle layer then dried, resulting in a dried aluminumparticle layer and a dried rear tabbing layer. During co-firing, bothdried layers are fired simultaneously in one step. The term “sequentialfiring” is used to describe the treatment for multilayer stacks that arefired multiple times. During sequential processing a metal particlepaste is applied on a substrate, dried and then fired. An intercalationpaste is then applied on a portion of the dried and fired metal particlepaste (referred to as the metal particle layer). Then, the entiremultilayer stack is dried and fired for a second time. It should benoted that embodiments of the invention that describe co-firedmultilayer stacks or structures are also appropriate for multilayerstacks or structures that have been fired sequentially.

The term “intercalation” is used herein to describe penetration into aporous material. In the context of the embodiments described herein, theterm “intercalation” describes the penetration of material fromintercalating particles in an intercalation layer into adjacent porousdry metal particle layer(s) during the firing process, which results inan intercalating particle material coating (partial or full) on at leasta portion of the metal particles. The term “modified metal particlelayer” is used herein to describe such a fired metal particle layer intowhich material from intercalating particles has penetrated.

In describing the relationship between adjacent layers, the preposition“on” is used herein to mean that the layers may or may not be in directphysical contact with one another. For example, to say that a layer ison a substrate is to say that the layer is positioned either directlyadjacent to or indirectly above or adjacent to the substrate. To saythat a particular layer is indirectly above or adjacent to a substrateis to say that there may or may not be one or more additional layersbetween the particular layer and the substrate. In describing therelationship between adjacent layers, the preposition “directly on” isused herein to mean that the layers are in direct physical contact withone another. For example, to say that a layer is directly on a substrateis to say that the layer is positioned directly adjacent to thesubstrate.

When a metal particle layer consists mainly of particles of metal A, itcan be referred to as a “metal A particle layer.” For example, when ametal particle layer consists mainly of aluminum particles, it can bereferred to as an aluminum particle layer. When a modified metalparticle layer consists mainly of particles of metal A, it can bereferred to as a “metal A modified particle layer.” For example, when amodified metal particle layer consists mainly of aluminum particles, itcan be referred to as a modified aluminum particle layer.

The term “solderable surface” is well known in the art. “Solderablesurface” refers to a surface that can be soldered to a soldering ribbon.A person with ordinary skill in the art is familiar with the concept ofa solderable surface. Examples of materials that create a solderablesurface include, but are not limited to, tin, cadmium, gold, silver,palladium, rhodium, copper, zinc, lead, nickel, alloys thereof,combinations thereof, composites thereof, and mixtures thereof. In oneembodiment, a surface is solderable when at least 70 wt % of the surfaceconsists of a material such as silver, gold, platinum, palladium,rhodium, and alloys, composites, and other combinations thereof.

Particles described herein can exhibit a variety of shapes, sizes,specific surface areas, and oxygen contents. Particles may bespheroidal, acicular, angular, dendritic, fibrous, flaky, granular,irregular, and nodular as defined by ISO 3252. It should be understoodthat the term “spherical” is used herein to refer to generally sphericalshapes and may include spheroidal, granular, nodular, and sometimesirregular shapes. The term “flake” refers to flaky, and sometimesangular, fibrous, and irregular shapes. The term “elongated” refers toacicular, and sometimes angular, dendritic, fibrous, and irregularshapes as defined by ISO 3252:1999. Particle shape, morphology, size,and size distribution often depend on synthesis technique. A group ofparticles may include a combination of particles of different shapes andsizes.

Particles that are spherical or elongated are typically described bytheir D50, specific surface area and particle size distribution. The D50value is defined as the value at which half of the particle populationhas a diameter below and half the particle population has a diameterabove the value. Measuring a particle diameter distribution is typicallyperformed with a laser diffraction particle size analyzer such as theHoriba LA-950. For example, spherical particles are dispersed in asolvent in which they are well separated and the scattering oftransmitted light is directly correlated to the size distribution fromsmallest to largest dimensions. A common approach to express laserdiffraction results is to report the D50 values based on volumedistributions. The statistical distribution of particle sizes can alsobe measured using a laser diffraction particle size analyzer. It iscommon for precious metal particles to have either a unimodal or amultimodal particle size distribution. In a unimodal distribution,particle size is monodispersed and the D50 is in the center of a singledistribution. Multimodal particle size distributions have more than onemode (or peak) in the particle size distribution. Multimodal particlesize distributions can increase the tap density of a powder, whichtypically results in a higher green film density.

In some embodiments of the invention, particles may have a flake orelongated shape as defined above. The flake may have a diameter between1 μm and 100 μm or between 1 μm and 15 μm and a thickness between 100 nmand 500 nm. An elongated shape may have a diameter between 200 nm and1000 nm and a length greater than 1 μm. In another embodiment of theinvention, there is no limitation on the shape of the particles; anyparticle shape can be used, as long as its largest dimension is nolarger than 50 μm, 5 μm, or 1 μm.

The specific surface area of particles can be measured using the BET(Brunauer-Emmett-Teller) method according to DIN ISO 9277, 2003-05. Thespecific surface area of the particles disclosed herein, and inparticular of the silver and bismuth particles is determined by thefollowing test method: BET measurement performed using TriStar 3000(from Micromeritics Instrument Corporation) which operates based on thephysical adsorption analytical technique. Sample preparation includesdegassing to remove adsorbed molecules. Nitrogen is the analysis gas andhelium is used to determine the void volume of the sample tube.Micromeritics provides silica alumina for use as a reference material,along with preparation procedures and testing conditions. Themeasurement begins by adding a known mass of the reference material to asample tube and mounting the sample tube on the BET apparatus manifold.The thermally-stable dosing manifold, sample tubes, and dedicated tubefor measuring saturation pressure (P_(o)) are evacuated. When sufficientvacuum is achieved, the manifold is filled with helium (a non-absorbinggas) and the sample port is opened to determine the warm free space ofthe sample at room temperature. The sample tube with the referencematerial are submerged in liquid nitrogen and cooled to around 77 K, andfree space analysis is once again conducted. Saturation pressure of theadsorptive is measured using P_(o) tube, followed by nitrogen dosinginto the manifold above atmospheric pressure. Pressure and temperatureof nitrogen are recorded, and then the sample port is open to letnitrogen adsorb onto the sample. After some time, the port is closed toallow adsorption to reach equilibrium. The amount adsorbed is thequantity of nitrogen removed from the manifold minus any residualnitrogen in the sample tube. Measured points along the adsorptionisotherm are used to calculate the specific area in m²/g for thereference material; this procedure is repeated with any sample ofinterest such as the particles described herein.

Particles described herein have salient thermal properties: meltingpoint and/or softening point, both of which depend on the crystallinityof the material. The melting point of particles can be determined bydifferential scanning calorimetry with a DSC 2500 differential scanningcalorimeter made by TA Instruments and using the method described inASTM E794-06 (2012). The melting point of crystalline materials can alsobe determined using a heating stage and x-ray diffraction. As acrystalline material is heated above its melting point, diffractionpeaks begin to disappear. The softening point is the temperature atwhich an amorphous, or glassy particle begins to soften. The softeningpoint of a glass particle may be determined using a dilatometer. Thesoftening point may also be obtained by the fiber elongation methoddescribed in ASTM C338-57.

Materials for Making Fired Multilayer Stacks

In one embodiment of the invention, a substrate, metal particle paste,and intercalation paste form a fired multilayer stack. The substrate maybe a solid, planar, or rigid material. In one embodiment, a substrateincludes at least one material selected from the group consisting ofsilicon, silicon dioxide, silicon carbide, aluminum oxide, sapphire,germanium, gallium arsenide, gallium nitride and indium phosphide. Suchsubstrates are commonly used for deposition of layers that make uptransistors, light emitting diodes, integrated circuits and photovoltaiccells. The substrate may also be electronically conductive and/orflexible. In another embodiment, the substrate includes at least onematerial selected from the group consisting of aluminum, copper, iron,nickel, titanium, steel, zinc, and alloys, composites, and othercombinations thereof.

In one embodiment of the invention, metal particle pastes include metalparticles and organic vehicle. In one arrangement, metal particle pastesalso include an inorganic binder such as a glass frit. In onearrangement, a common, commercially available metal particle paste isused. Metal pastes containing aluminum commonly used on silicon solarcells are sold by Ruxing Technology (e.g., RX8252H1), Monocrystal (e.g.,EFX-39), and GigaSolar Materials (e.g., M7). Metal particles may includeat least one of aluminum, copper, iron, nickel, molybdenum, tungsten,tantalum, titanium, or alloys, composites, or other combinationsthereof. In various arrangements, metal particles have a D50 between 100nm and 100 μm, between 500 nm and 50 μm, between 500 nm and 20 μm, orany range subsumed therein. The metal particles may have a spherical,elongated or flake shape and may have a unimodal or multimodal sizedistribution. Glass frit may be included in metal particle pastes insmall quantities (i.e., less than 5 wt %). In one embodiment, a metalparticle paste comprises 70 wt % to 80 wt % aluminum particles, lessthan 2 wt % glass frit, and organic vehicle.

In one embodiment of the invention, an intercalation paste includesprecious metal particles, intercalating particles and an organicvehicle. The term “solids loading” can be used in connection with pastesto describe the amount or proportion of precious metal and intercalatingparticle solids in a paste. The pastes described herein also include anorganic vehicle, although that may not always be stated explicitly.

Intercalation Paste Components

In one embodiment of the invention, precious metal particles, asdescribed herein, comprise at least one material selected from the groupconsisting of gold, silver, platinum, palladium, and rhodium, andalloys, composites, or other combinations thereof. In one embodiment,precious metal particles comprise between 10 wt % and 70 wt % of thepaste. In various embodiments, precious metal particles have a D50between about 100 nm and 50 μm, between 300 nm and 10 μm, between 300 nmand 5 μm, or any range subsumed therein. In various embodiments,precious metal particles have a specific surface area ranging from about0.4 to 7.0 m²/g or from about 1 to 5 m²/g or any range subsumed therein.The precious metals may have up to 2 wt % oxygen content; the oxygen maybe mixed uniformly throughout the particle or the oxygen may be found asan oxide shell that has a thickness up to 500 nm. The precious metalparticles may have a spherical, elongated or flake shape and have aunimodal or multimodal size distribution. Silver particles are commonlyused in metallization pastes in the solar industry. In an exemplaryembodiment, at least some precious metal particles are silver with a D50between 300 nm and 2.5 μm and a specific surface area between 1 and 3m²/ g.

The term “intercalating particles” is used to describe particles thatcan deform when heated, and, when placed adjacent to a porous layer ofother metal particles, can at least partially intercalate into theporous metal particle layer and phase separate from the other metalparticles under the influence of heat. In various arrangements,intercalating particles have a D50 between 50 nm and 50 μm, between 50nm and 10 μm, between 300 nm and 5 μm, or any range subsumed therein. Inone embodiment, intercalating particles have a D50 between 300 nm and 3μm. In various embodiments, intercalating particles have a specificsurface area ranging from about 0.1 to 6 m²/g, about 0.5 to 3 m²/ g orabout 0.5 to 4 m²/g or any range subsumed therein. According to oneembodiment, intercalating particles are flakes and have a specificsurface area of about 1.0 to 3.0 m²/g. The intercalating particles mayhave a spherical, elongated or flake shape and may have a unimodal ormultimodal size distribution.

There are three groups of particles that can be used as intercalatingparticles: low temperature base metal particles (LTBMs), crystallinemetal oxide particles, and glass frit particles. In some arrangements,intercalating particles consist solely of low temperature base metalparticles, or of crystalline metal oxide particles, or of glass fritparticles. In another arrangement, intercalating particles are mixturesof particles from two or more of these groups. It is desirable that theelements of the intercalating particles have low solubility with and donot alloy with elements in adjacent metal particle layers.

In one embodiment, an intercalating particle is a low temperature basemetal particle. The term “low temperature base metal particles” (LTBMs)is used herein to describe particles that consist exclusively oressentially of any base metal or metal alloy that has a low-temperaturemelting point, i.e., a melting point below 450° C. In some arrangements,LTBMs also contain up to 2 wt % oxygen; the oxygen may be mixeduniformly throughout the particle, or the oxygen may be found in anoxide shell that has a thickness up to 500 nm and coats or partiallycoats the particle. In some arrangements, the melting point of LTMBs iseven lower, such as below 350° C. or below 300° C. In one embodiment ofthe invention, LTBMs are made exclusively or essentially of bismuth,tin, tellurium, antimony, lead, or alloys, composites, or othercombinations thereof. In one embodiment, intercalating particles containonly bismuth and have a D50 between 1.5 and 4 μm and a specific surfacearea between 1 and 2 m²/g.

In another embodiment, a LTBM intercalating particle is a bismuth coreparticle surrounded by a metal or metal oxide shell. In anotherembodiment, a LTBM intercalating particle is a bismuth core particlesurrounded by a single shell made of silver, nickel, nickel alloy suchas nickel boron, tin, tellurium, antimony, lead, molybdenum, titanium,composites, and/or other combinations thereof. In another embodiment, aLTBM intercalating particle is a bismuth core particle surrounded by asingle shell that is an oxide of silicon, an oxide of magnesium, anoxide of boron, or any combination thereof. Any of these shells may havea thickness that ranges from 0.5 nm to 1 μm, or 0.5 nm to 200 nm, or anyrange subsumed therein.

In another embodiment, an intercalating particle is a crystalline metaloxide particle. A metal oxide is any compound that has at least oneoxygen atom (anion with an oxidation state of −2) and at least one metalatom. Many metal oxides contain multiple metal atoms that may all be thesame or that may include a variety of metals. A wide range of metal tooxygen atom ratios is possible, as would be understood by a person withordinary skill in the art. When metal oxides form ordered, periodicstructures, they are crystalline. Such crystalline metal oxides candiffract x-ray radiation in a pattern of peaks of varying intensitiescharacteristic of their crystal structure. In one embodiment, acrystalline metal oxide particle consists exclusively or essentially ofan oxide of at least one of the following metals: bismuth, tin,tellurium, antimony, lead, vanadium, chromium, molybdenum, boron,manganese, cobalt, and alloys, composites or other combinations thereof.

For the structures disclosed herein and described in more detail below,as crystalline metal oxide particles are heated, it is useful if theybegin to melt (i.e., reach their melting point (T_(M))) at a lowertemperature than the temperature at which significant interdiffusion canoccur within the structure between or among metal particles of differentcompositions. The melting point of crystalline materials in a mixedlayer can be determined using a heating stage and x-ray diffraction; asa sample is heated above its melting point, diffraction peaks diminishand then disappear. In some exemplary embodiments, boron (III) oxide(B₂O₃ T_(M)=450° C.), vanadium (V) oxide (V₂O₅ T_(M)=690° C.), tellurium(IV) oxide (TeO₂ T_(M)=733° C.), and bismuth (III) oxide (Bi₂O₃T_(M)=817° C.) can deform during the firing process and intercalate intoan adjacent porous metal particle layer, creating the modified metalparticle layer. In an exemplary embodiment, the intercalating particlesare crystalline bismuth oxide with a D50 between 50 nm and 2 μm and aspecific surface area between 1 and 5 m²/g. In another embodiment, acrystalline metal oxide particle also contains a small amount (i.e.,less than 10 wt %) of one or more additional elements that may adjustthe melting point of the particle. Such additional elements include, butare not limited to, silicon, germanium, lithium, sodium, potassium,cesium, magnesium, calcium, strontium, barium, zirconium, hafnium,vanadium, niobium, chromium, molybdenum, manganese, rhenium, iron,cobalt, zinc, cadmium, gallium, indium, carbon, nitrogen, phosphorous,arsenic, antimony, sulfur, selenium, fluorine, chlorine, bromine,iodine, lanthanum, and cerium.

In another embodiment, an intercalating particle is a glass fritparticle. In one embodiment, a glass frit particle consists exclusivelyor essentially of the combination of oxygen and at least one of thefollowing elements: silicon, boron, germanium, lithium, sodium,potassium, cesium, magnesium, calcium, strontium, barium, zirconium,hafnium, vanadium, niobium, chromium, molybdenum, manganese, rhenium,iron, cobalt, zinc, cadmium, gallium, indium, carbon, tin, lead,nitrogen, phosphorous, arsenic, antimony, bismuth, sulfur, selenium,tellurium, fluorine, chlorine, bromine, iodine, lanthanum, cerium,oxygen, and alloys, composites, and other combinations thereof. It isuseful if the glass frit has a softening point below 900° C. or below800° C. in order to effectively deform during firing. In an exemplaryembodiment, the intercalating particles are bismuth silicate glass fritparticles with a D50 between 50 nm and 2 μm and a specific surface areabetween 1 and 5 m²/g.

The term “organic vehicle” describes mixtures or solutions of organicchemicals or compounds that assist in dissolving, dispersing and/orsuspending solid components in a paste. For the intercalation pastesdescribed herein, many different organic vehicle mixtures may be used.Such organic vehicles may or may not contain thixotropes, stabilizers,emulsifiers, thickeners, plasticizers, surfactants and/or other commonadditives.

Components of organic vehicles are well known to a person with ordinaryskill in the art. Major constituents of organic vehicles include one ormore binders and one or more solvents. Binders may be polymeric ormonomeric organic compounds, or “resins,” or a mixture of the two.Polymeric binders may have a variety of molecular weights and a varietyof polydispersity indices. Polymeric binders may include a combinationof two different monomeric units that are known as copolymers, in whichmonomeric units may either alternate singly or in large blocks (blockcopolymers). Poly-sugars are commonly used polymeric binders andinclude, but are not limited to, alkyl cellulose and alkyl derivativessuch as methyl cellulose, ethyl cellulose, propyl cellulose, butylcellulose, ethylhydroxyethyl cellulose, derivatives and mixturesthereof. Other polymeric binders include, but are not limited to,polyesters, polyethylenes, polypropylenes, polycarbonates,polyurethanes, polyacrylates (including polymethacrylate andpolymethymethacrylate), polyvinyls (including polyvinylchloride,polyvinylpyrrolidone, polyvinylbutyral, polyvinyl acetate), polyamides,polyglycols (including polyethylene glycol), phenolic resins,poly-terpenes, derivatives and combinations thereof. An organic vehiclebinder may include between 1 and 30 wt % binder.

Solvents are organic species that are usually removed from pastes duringprocessing by thermal means such as vaporization. In general, solventsthat can be used in the pastes described herein include, but are notlimited to, polar, non-polar, protic, aprotic, aromatic, non-aromatic,chlorinated, and non-chlorinated solvents. Examples of solvents that canbe used in the pastes described herein include, but are not limited to,alcohols, di-alcohols (including glycols), multi-alcohols (includingglycerol), mono- and poly-ethers, mono- and poly-esters, alcohol ethers,alcohol esters, mono-and di-substituted adipate esters, mono- andpoly-acetates, ether acetates, glycol acetates, glycol ethers (includingethylene glycol monobutyl ether, diethylene glycol monobutyl ether, andtriethylene glycol monobutyl ether), glycol ether acetates (includingethylene glycol monobutyl ether acetate), linear or branched saturatedand unsaturated alkyl chains (including butane, pentane, hexane, octane,and decane), terpenes (including alpha-, beta-, gamma-, and4-terpineol), 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (alsoknown as texanol™), 2-(2-ethoxyethoxy)ethanol (also known as carbitol™),derivatives, combination, and mixtures thereof.

In one arrangement, an organic vehicle contains between 70 and 100 wt %solvent. The proportion and composition of the binder, solvent, and anyadditives may be adjusted to achieve a desired dispersion or suspensionof paste particles, a desired carbon content, and/or desired rheologicalproperties, as would be understood by a person with ordinary skill inthe art. For example, paste rheology can be modified by addingthixatropic agents such as Thixatrol Max®. In another example, thecarbon content of the organic vehicle may be increased or decreased bymodifying the binder and thixatropic agent and taking into account peakfiring temperature, firing profile, and airflow that will occur duringthermal annealing. Minor additives may also be included. Such additivesinclude, but are not limited to, thixotropic agents and surfactants.Such additives are well known in the art and useful amounts of suchcomponents can be determined through routine experimentation to maximizedevice efficiency and reliability. In one embodiment of the invention, ametallization paste has a viscosity between 10,000 and 200,000 cP at 25°C. and at a sheer rate of 4 sec⁻¹ as measured using atemperature-controlled Brookfield RVDV-II+Pro viscometer.

Intercalation Paste Formulations

Exemplary composition ranges of intercalation pastes are shown in TableI, according to some embodiments of the invention. In variousembodiments, the intercalation paste has a solids loading between 30 wt% and 80 wt %, precious metal particles make up between 10 wt % and 70wt % of the intercalation paste, intercalating particles make up atleast 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, or 40 wt % of theintercalation paste, and the ratio of intercalating particles toprecious metal particles is at least 1:5 by weight. In an exemplaryembodiment, the precious metal particle content is 50 wt %, and theintercalating particles make up at least 10 wt % of the intercalationpaste. In various embodiments, the ratio of intercalating particles toprecious metal particles in the intercalation paste is at least 1:5, or2:5, or 3:5, or 1:1, or 5:2 by weight.

TABLE I Intercalation Paste Formulations by Weight Percent (wt %)Precious Metal Intercalating Organic Paste Type Particles ParticlesVehicle Intercalation Paste (range I) 10-70 10-50 20-70 IntercalationPaste (range II) 20-50 10-35 30-60 Intercalation Paste A 50 12.5 37.5Intercalation Paste B 45 30 25 Intercalation Paste C 45 30 25Intercalation Paste D 30 20 50

In one embodiment of the invention, for solar cell applications, anintercalation paste contains between 20 and 50 wt % precious metalparticles (i.e., intercalation paste range II in Table I) and between 10and 35 wt % intercalating particles, which may include LTBM, crystallinemetal oxides, glass frits, or a combination thereof. In one embodiment,the intercalating particles are metallic bismuth particles.Intercalation paste A (Table I) may contain 50 wt % silver particles,12.5 wt % bismuth particles, and 37.5 wt % organic vehicle, resulting ina 1:4 ratio (by weight) of intercalating particles to precious metalparticles. Intercalation paste C (Table I) may contain 45 wt % silverparticles, 30 wt % bismuth particles, and 25 wt % organic vehicle,resulting in a 1:1.5 ratio (by weight) of intercalating particles toprecious metal particles. When the intercalation paste comprises silverand bismuth particles, the notation Ag:Bi is used.

In another embodiment, the intercalating particles are glass fritparticles. Intercalation paste B (Table I) may contain 45 wt % silverparticles, 30 wt % bismuth-based glass frit particles, and 25 wt %organic vehicle, resulting in a 1:1.5 ratio (by weight) of intercalatingparticles to precious metal particles. In another embodiment,intercalating particles are a mixture of LTBMs, crystalline metal oxideparticles, and glass frit particles. Intercalation paste D (Table I) maycontain 30 wt % silver particles, 15 wt % metallic bismuth particles, 5wt % high-lead-content glass frit particles and 50 wt % organic vehicle.The formulation of the intercalation pastes can be adjusted to achieve adesired bulk resistance, contact resistance, layer thickness, and/orpeel strength for a particular metal layer.

In another embodiment of the invention, a method of forming anintercalation paste includes the steps of providing precious metalparticles, providing intercalating particles, and mixing the preciousmetal particles and intercalating particles together in an organicvehicle. In one arrangement, the intercalating particles are added tothe organic vehicle and mixed in a planetary mixer (e.g., Thinky AR-100)and then the precious metal particles (and additional organic vehicle,if desired) are added and mixed in the planetary mixer. Theintercalation paste may or may not then be milled, for example, by usinga three roll mill (e.g., Exakt 50 I). In one arrangement, theintercalation paste contains between 10 and 70 wt % precious metalparticles and more than 10 wt % intercalating particles.

Methods of Forming a Fired Multilayer Stack

In one embodiment of the invention, a fired multilayer stack includes asubstrate on which there is at least one metal particle layer and atleast one intercalation layer. In one embodiment, a fired multilayerstack is formed using a co-firing process that involves the followingsteps: applying a metal particle layer onto a substrate surface, dryingthe metal particle layer, applying an intercalation layer directly on aportion of the dried metal particle layer, drying the intercalationlayer, and then co-firing the multilayer stack. In another embodiment, afired multilayer stack is formed using a sequential firing process thatinvolves the following steps: applying a metal particle layer onto asubstrate surface, drying the metal particle layer, firing the metalparticle layer, applying an intercalation layer directly on a portion ofthe fired metal particle layer, drying the intercalation layer and thenfiring the multilayer stack. In one embodiment, during firing, a portionof the intercalation layer penetrates into the metal particle layer,thereby transforming the metal particle layer into a modified metalparticle layer. In some embodiments, each applying step involves amethod selected independently from the group consisting of screenprinting, gravure printing, spray deposition, slot coating, 3D printingand inkjet printing. In one embodiment, a metal particle layer isapplied by screen printing a metal particle paste on a portion of asubstrate, and an intercalation layer is applied by screen-printing anintercalation paste directly on a portion of a metal particle layerafter it has been dried. In one embodiment, a portion of a substratesurface is covered by at least one dielectric layer and a metal particlelayer is applied onto a portion of the dielectric layer.

Dried and Fired Multilayer Stack Morphology

FIG. 1 is a schematic cross-section drawing that shows a multilayerstack 100 before it is co-fired, according to an embodiment of theinvention. A dried metal particle layer 120, is directly on a portion ofa substrate 110. An intercalation layer 130, made up of intercalatingparticles and precious metal particles, as described above, is directlyon a portion of the dried metal particle layer 120. In variousembodiments of the invention, the intercalation layer 130 has an averagethickness between 0.25 μm and 50 μm, between 1 μm and 25 μm, between 1μm and 10 μm, or any range subsumed therein. In one embodiment of theinvention, the intercalation layer 130 includes precious metalparticles, intercalating particles, and optional organic binder (whichmay remain in the intercalation layer 130 after drying). Prior toco-firing, the precious metal particles and intercalating particles maybe homogeneously distributed within the intercalation layer 130. In onearrangement, the precious metal particles and the intercalatingparticles are not deformed after drying (and prior to firing), retainingtheir original size and shape.

In one embodiment of the invention, the dried metal particle layer 120is porous and includes at least one of aluminum, copper, iron, nickel,molybdenum, tungsten, tantalum, titanium, and alloys, composites, andother combinations thereof. In one arrangement, prior to co-firing, thedried metal particle layer 120 contains metal particles, and may or maynot contain organic binder, and may or may not contain non-metallicparticles such as glass frit. The metal particles are typically notdeformed after drying (and prior to firing), retaining their originalsize and shape.

During firing, the intercalating particles from the intercalation layer130 intercalate into a portion of the dried metal particle 120 layeradjacent to (shown as below in FIG. 1) the intercalation layer 130. Theportion of the dried metal particle layer 120 adjacent to theintercalation layer 130 and into which the intercalating particlematerial penetrates is called the “modified metal particle layer” forthe purposes of this disclosure. After firing, the remaining portion ofthe dried metal particle layer 120 that is not adjacent to theintercalation layer and into which no, or only trace amounts ofintercalating particle material penetrates, is referred to as the “metalparticle layer” for the purposes of this disclosure. In one arrangement,during firing, particles in the dried metal particle layer 120 maysinter or melt, so that the metal particle layer has a differentmorphology and less porosity than the dried metal particle layer 120.The changes that occur during firing and the fired multilayer stackstructure are discussed in more detail below.

FIG. 2 is a schematic cross-section drawing that shows a firedmultilayer stack 200 (the structure 100 of FIG. 1 after it has beenfired), according to an embodiment of the invention. The firedmultilayer stack 200 includes a modified (due to firing) metal particlelayer 222 adjacent to at least a portion of a substrate 210 and amodified (due to firing) intercalation layer 230 adjacent to themodified metal particle layer 222. During firing, at least a portion ofthe precious metal particles and intercalating particles in theintercalation layer (shown as 130 in FIG. 1 before firing) form phasesthat phase separate from each other. The precious metal particles maysinter or melt, changing the morphology and reducing the porosity of themodified intercalation layer 230. At least a portion of theintercalating particles melt and flow, or intercalate, into the adjacentmodified metal particle layer 222 as at least a portion of the preciousmetal particles (which may sinter or melt) move toward a solderablesurface 230S of the modified intercalation layer 230. The modified metalparticle layer 222 includes metal particles into which material from theintercalating particles in the intercalation layer (shown as 130 in FIG.1 before firing) has penetrated, altering the material properties of aportion of the dried metal particle layer (shown as 120 in FIG. 1 beforefiring) to form the modified metal particle layer 222. The material fromthe intercalating particles may connect loosely packed metal particlesin the modified metal particle layer 222, or it may coat metal particlesthat are already in contact with one another in the modified metalparticle layer 222.

In some arrangements, there are also metal particle regions 220 intowhich little or only trace amounts of intercalation particle materialhave penetrated. In one arrangement, a metal particle layer 220, whichis not in direct contact with the modified intercalation layer 230, doesnot contain an increased concentration of elements from theintercalation particles. In some arrangements, the metal particle layer220 and the modified metal particle layer 222 form a compound(s) withthe substrate 210 or dope the substrate 210 during co-firing (notshown). Although FIG. 2 indicates sharp boundaries between the metalparticle layer regions 220 and the modified metal particle layer 222, itshould be understood that the boundary is not generally sharp. In somearrangements, the boundary is determined by the extent of lateraldiffusion of the modified intercalation layer 230 material into themetal particle layer 220 during co-firing.

In some embodiments of the invention, the materials in the modifiedintercalation layer 230 in FIG. 2 are phase separated into a phase thatcontains material from the intercalating particles and a phase thatcontains the precious metal. FIG. 3 is a schematic cross-section drawingthat shows a fired multilayer stack 390 (equivalent to structure 200 ofFIG. 2) and in which the intercalation layer 330 has phase separated.The fired multilayer stack 390 (within region 350 only) includes amodified (due to firing) metal particle layer 322 in region 350 betweena portion of a substrate 300 and a modified (due to firing)intercalation layer 330. A metal particle layer 320 that contains metalparticles 392 is on the substrate 300 adjacent to the multilayer stackregion 350.

The modified intercalation layer 330 contains two phases: a preciousmetal phase 335 and an intercalation phase 333 and has a solderablesurface 335S. Most (at least more than 50%) of the solderable surface ismade up of the precious metal phase 335. In some arrangements, theprecious metal phase 335 and intercalation phase 333 do not phaseseparate completely during firing, so that there is also some of theintercalation phase 333 at the solderable surface 335S. The modifiedmetal particle layer 322 contains metal particles 392 and a portion ofmaterial from the intercalation phase 333. There is an interface322Ibetween the modified intercalation region 330 and the adjacent metalparticles 392 in the modified metal particle layer 322. Te interface322Imay not be smooth and is dependent on the size and shape of themetal particles 392, as well as the firing conditions. In embodimentswhere optional glass frit has been included in the dry metal particlelayer (120 in FIG. 1) before firing, the modified metal particle layer322 and the metal particle layer 320 may also contain a small amount ofglass frit (not shown), which makes up less than 3 wt % of the layer.

In other embodiments, the materials in the modified intercalation layer230 in FIG. 2 are phase separated into to form a layered structure. FIG.4 is a schematic cross-section drawing that shows a fired multilayerstack 400 (equivalent to the structure 200 of FIG. 2) including anintercalation layer with two sublayers. The fired multilayer stack 400(within region 450 only) includes a modified (due to firing) metalparticle layer 422 in region 450 between a portion of a substrate 410and a modified (due to firing) intercalation layer 430. A metal particlelayer 420 that contains metal particles 402 is on the substrate 410adjacent to the multilayer stack region 450.

The modified intercalation layer 430 contains two sublayers: anintercalation sublayer 433 directly on the modified metal particle layer422 and a precious metal sublayer 435 directly on the modifiedintercalation layer 433. The precious metal sublayer 435 has asolderable surface 435S. The modified metal particle layer 422 containsmetal particles 402 and some material 403 from the intercalationsublayer 433. There is an interface 422I between the modifiedintercalation layer 430 (or the modified intercalation layer 433) andthe top-most metal particles 402 in the modified metal particle layer422. In embodiments where optional glass frit has been included in thedry metal particle layer (120 in FIG. 1) before firing, the modifiedmetal particle layer 422 and the metal particle layer 420 may alsocontain a small amount of glass frit (not shown), which makes up lessthan 3 wt % of the layer.

Cross-sectional SEM imaging was used to identify the layers and measurethe layer thicknesses in a multilayer stack. Average layer thicknessesof layers in a fired multilayer stack were obtained by averaging atleast ten thickness measurements, each separated by at least 10 μm,across a cross-sectional image. In various embodiments of the invention,a metal particle layer (such as 220 in FIG. 2) has an average thicknessbetween 0.5 μm and 100 μm, between 1 μm and 50 μm, between 2 μm and 40μm, between 20 μm and 30 μm, or any range subsumed therein. Such a metalparticle layer on a substrate is typically smooth with a minimum andmaximum layer thickness within 20% of the average metal particle layerthickness over a 1×1 mm area. In addition to cross-section SEM, thelayer thickness and variation over the described area can be accuratelymeasured using an Olympus LEXT OLS4000 3D Laser Measuring Microscopeand/or a profilometer such as a Veeco Dektak 150.

In an exemplary embodiment, a metal particle layer (such as 220 in FIG.2) is made of sintered aluminum particles and has an average thicknessof 25 μm. The porosity of the metal particle layer can be measured usinga mercury porosimeter such as a CE instrument Pascal 140 (low pressure)or Pascal 440 (high pressure) in a range between 0.01 kPa and 2 Mpa.Fired metal particle layers may have porosities between 1% and 50%,between 2% and 30%, between 3% and 20%, or any range subsumed therein.Fired metal particle layers made of aluminum particles and used in solarapplications may have a porosity between 10% and 18%.

The thicknesses of an intercalation sublayer and of the precious metalsublayer, such as those shown schematically as 433 and 435,respectively, in FIG. 4, were measured in an actual multilayer stackusing cross-sectional SEM/EDX. The sublayers were distinguished in SEMdue to the differences in contrast between the intercalation andprecious metal phases. EDX mapping was used to identify the location ofthe interface shown as 432I in FIG. 4. In various embodiments, theprecious metal sublayer has a thickness between 0.5 μm and 10 μm,between 0.5 μm and 5 μm, between 1 μm and 4 μm, or any range subsumedtherein. In various embodiments, the intercalation sublayer has athickness between 0.01 μm and 5 μm, between 0.25 μm and 5 μm, between0.5 μm and 2 μm, or any range subsumed therein.

In one embodiment of the invention, a modified intercalation layercontains two phases: a precious metal phase and an intercalation phaseSuch a structure was shown in detail in FIG. 4. Typically, theintercalation phase is not solderable, so it is useful if the solderablesurface 230S contains mostly the precious metal phase to ensuresolderability. In various arrangements, the solderable surface containsmore than 50%, more than 60%, or more than 70% precious metal phase. Inone arrangement, the solderable surface of the modified intercalationlayer contains mostly precious metal(s). Plan view EDX was used todetermine the concentration of elements on the surface of the modifiedintercalation layer. SEM/EDX was performed with the equipment describedabove and at an accelerating voltage of 10 kV with a 7 mm sample workingdistance and 500 times magnification. In various embodiments, at least70 wt %, at least 80 wt %, at least 90 wt %, at least 95 wt % or atleast 98 wt % of the outside surface 230S of the modified intercalationlayer 230 contains one or more of gold, silver, platinum, palladium,rhodium, and alloys, composites, and other combinations thereof. Thefiring conditions, intercalating particle and precious metal particletypes and sizes all affect the degree of phase separation in modifiedintercalation layer morphology.

The modified metal particle layer (shown as 222 in FIG. 2) contains amuch higher concentration of the intercalating particle material thandoes the metal particle layer (shown as 220 in FIG. 2). A comparison ofEDX spectra taken from cross-sections of the modified metal particlelayer and from the metal particle layer in an actual multilayer stackcan be used to determine the concentration of material from the modifiedintercalation layer that has intercalated into the modified metalparticle layer. The SEM/EDX equipment described above, operated at 20 kVwith a sample working distance of 7 mm, was used to measure the ratio ofmetal from the intercalation particles (e.g., bismuth) to total metal(e.g., bismuth and aluminum) in a cross-section sample of the modifiedmetal particle layer. The weight ratio (intercalation metal to totalmetal) is referred to as the IM:M ratio. A baseline EDX analysis wasperformed in a region of the metal particle layer that was at least 500μm away from the modified metal particle layer to ensure reproduciblemeasurements. A second EDX spectrum was taken from the modified metalparticle layer, and the spectra were compared. In determining the IM:Mratio, only the peaks of the metallic elements were considered (i.e.,peaks from carbon, sulfur, and oxygen were ignored). When analyzing theratio, precious metals and any metallic elements from the substrate wereexcluded in order to prevent unreliable results. In an exemplaryembodiment, when the dry metal particle layer (shown as 120 in FIG. 1)contained aluminum particles and the intercalation layer 130 containedbismuth and silver particles, the metal particle layer (i.e., afterfiring) contained approximately 1 wt % bismuth and more than 98 wt %aluminum with a Bi:(Al+Bi) (IM:M) ratio of 1:99. Other intercalationmetal components make up less than 0.25 wt % of the modified metalparticle layer and are not considered in calculation the IM:M ratio. Invarious other embodiments, the IM:M ratio is 1:10⁶, 1:1000, 1:100, 1:50,1:25, or 1:10.

It should be noted that substrates may have some surface roughness,which may cause interfaces with them to also be rough. FIG. 5 is aschematic cross-section drawing that shows a portion of such a substrate510, a modified metal particle layer 522 and a modified intercalationlayer 530, according to an embodiment of the invention. There is anon-planar interface 501B between the substrate 510 and the modifiedmetal particle layer 522. There is a non-planar interface 522B betweenthe modified metal particle layer 522 and the modified intercalationlayer 530. Line 502 indicates the deepest incursion of the substrate 510into the modified metal particle layer 522. Line 504 indicates thedeepest incursion of the modified intercalation layer 530 into themodified metal particle layer 522. The region of the modified metalparticle layer 522 between the line 502 and the line 504 can be referredto as sampling region 522A. In determining the IM:M ratio in themodified metal particle layer 522, it can be useful to limit suchanalysis to the sampling region 522A in order to avoid spurious resultsdue to interface roughness.

In exemplary embodiments, the IM:M ratio in a modified metal particlelayer is 20% higher, 50% higher, 100% higher, 200% higher, 500% higher,or 1000% higher than in a metal particle layer (in regions at least 500μm away from the modified metal particle layer). In an exemplaryembodiment, an intercalation layer containing bismuth particles is on analuminum particle layer, and the modified metal particle layer (asanalyzed in a sampling region such as that shown as 522A in FIG. 5)contains 4 wt % bismuth and 96 wt % aluminum for a Bi:(Al+Bi) (or IM:M)ratio of 1:25. The Bi:(Al+Bi) ratio is 400% higher in the modified metalparticle layer than in the metal particle layer.

When an intercalation layer contains crystalline metal oxides and/orglass frits that contain more than one metal, the intercalation metalcomponents are quantified by EDX and summed to determine to IM:M ratio.For example, if a glass frit contains both bismuth and lead, then theratio is defined as (Bi+Pb):(Bi+Pb+Al).

In various embodiments, a fired multilayer stack also includes a solidcompound layer formed from interactions between metal particles in adried metal particle layer and a substrate during firing. The solidcompound layer can include, but is not limited to, alloys, eutectics,composites, mixtures, or combinations thereof. In one arrangement, amodified metal particle layer and a substrate form solid compoundregion(s) at their interface. The solid compound region(s) may containone or more alloys. The solid compound region(s) may be continuous (alayer) or semi-continuous. Depending on the composition of the substrateand metal particle layer, alloy(s) or other compounds that form mayinclude one or more of aluminum, copper, iron, nickel, molybdenum,tungsten, tantalum, titanium, silicon, oxygen, carbon, germanium,gallium, arsenic, indium and phosphorous. For example, aluminum andsilicon can form a eutectic above 660° C. which, upon cooling, resultsin a solid aluminum-silicon (Al—Si) eutectic layer at the siliconinterface. In an exemplary embodiment, the solid compound layer is asolid Al—Si eutectic layer formed on a portion of the silicon substrate.The formation and morphology of the solid Al—Si eutectic layer is wellknown in silicon solar cells. In another embodiment, a substrate isdoped with at least one of aluminum, copper, iron, nickel, molybdenum,tungsten, tantalum, titanium, and alloys, composites, and othercombinations thereof. In one example, aluminum is a p-type dopant insilicon and, during firing, aluminum from an aluminum particle layeradjacent to the substrate, provides more aluminum dopant to form ahighly p-type doped region in the silicon substrate, which is known asthe back-surface field.

Depending on the atmospheric conditions, intercalating particles mayundergo multiple phase changes as they melt and intercalate into amodified metal particle layer in a fired multilayer stack. Depending onthe materials in the modified metal particle layer and in the substrate,the intercalating particles may also form crystalline compounds as theyintercalates into the modified metal particle layer. Such crystallinecompounds can improve cohesion between metal particles in the modifiedmetal particle layer, prevent interdiffusion of certain elements, and/orreduce electrical contact resistance between metal layers in the firedmultilayer stack. In one embodiment, the modified intercalation layerand the modified metal particle layer contain crystallites made up ofbismuth and at least one other of oxygen, silicon and silver and alloys,composites, and other combinations thereof.

In one embodiment of the invention, a precious metal phase includes atleast one material selected from the group consisting of gold, silver,platinum, palladium, rhodium, and alloys thereof, composites thereof,and other combinations thereof. In one arrangement, the precious metalphase consists essentially of one or more of these materials. When oneof these materials makes up the majority of the precious metal phase,the precious metal phase is described as being rich in that material.For example, if a precious metal phase, precious metal layer, orprecious metal sublayer contains mostly silver, it can be referred to asilver-rich region, silver-rich layer, or silver-rich sublayer,respectively.

An intercalation phase contains elements from intercalating particlesand may also include elements from the outside environment (e.g.,oxygen) and small quantities of elements from precious metal particlesin an adjacent metal particle layer, and a nearby substrate, which havebeen incorporated during firing. A wide array of elements may be in theintercalation phase depending on whether low-temperature base metals,crystalline metal oxides and/or glass frits are used as intercalatingparticles. In one embodiment (when the intercalating particles are lowtemperature base metals exclusively), the intercalation phase containsat least one material selected from the group consisting of bismuth,boron, tin, tellurium, antimony, lead, oxygen, and alloys, composites,and other combinations thereof. In another embodiment (when theintercalating particles are crystalline metal oxides exclusively), theintercalation phase contains at least one material selected from thegroup consisting of oxides of bismuth, tin, tellurium, antimony, lead,vanadium chromium, molybdenum, boron, manganese, cobalt, and alloys,composites, and other combinations thereof. In another embodiment (whenthe intercalating particles are glass frits exclusively), theintercalation phase contains oxygen and at least one of the followingelements: silicon, boron, germanium, lithium, sodium, potassium, cesium,magnesium, calcium, strontium, barium, zirconium, hafnium, vanadium,niobium, chromium, molybdenum, manganese, rhenium, iron, cobalt, zinc,cadmium, gallium, indium, carbon, tin, lead, nitrogen, phosphorous,arsenic, antimony, bismuth, sulfur, selenium, tellurium, fluorine,chlorine, bromine, iodine, lanthanum, cerium, and alloys, composites,and other combinations thereof. When one of these materials makes up themajority of the intercalation region, the intercalation region isdescribed as being rich in that material. For example, if anintercalation region, intercalation layer, or intercalation sublayercontains mostly bismuth, it can be referred to a bismuth-rich region,bismuth-rich layer, or bismuth-rich sublayer, respectively.

Examples of Fired Multilayer Stacks and Applications

Metal particle layers that contain mostly aluminum, copper, iron,nickel, molybdenum, tungsten, tantalum, and titanium are not solderablewith mildly activated (RMA) fluxes and tin-based solders after firing.Yet, in solar cells and other devices, it is highly desirable to solderribbons to make electrical connections with metal particle layers suchas aluminum particle layers. As is disclosed herein, inventiveintercalation pastes that contain precious metals such as silver andgold can be used on metal particle layers and fired in air to producesurfaces that are highly solderable. This is in contrast to otherattempts to increase the solderability of metal particle layers byadding precious metals, as precious metals usually interdiffuse with themetal particle layers (e.g., aluminum) upon firing in multilayer stacks,resulting in a solderable surface that contains too little preciousmetal to solder well. For example, firing a layer of commerciallyavailable silver rear tabbing paste that contains less than 10 wt %glass frit on an aluminum particle layer does not result in a solderablesurface. Such layers experience significant silver-aluminuminterdiffusion during the firing step and the resulting silver aluminidesurface is not solderable

Intercalation layers, as disclosed herein, can be used to modify thematerial properties of metal particle layers in order to 1) to blockdiffusion of precious metals and provide a solderable surface, 2)mechanically strengthen the metal particle layer and 3) assist inetching layers beneath the metal particle layers. In one embodiment ofthe invention, multilayer stacks were formed using intercalation pastesthat contain precious metal particles made of silver and intercalatingparticles made of bismuth metal or bismuth based glass frits, andadjacent metal particle layers that contain aluminum particles. Firedmultilayer stacks were formed by screen printing aluminum pastes(commonly used for solar cell applications) on a bare silicon wafer,drying the sample at 250° C. for 30 seconds, screen printing theintercalation paste on a portion of the dried aluminum particle layer,drying the sample at 250° C. for 30 seconds and co-firing the sampleusing a spike fire profile with a peak temperature between 700° C. and820° C. and a ramp up and cool down rate greater than 10° C./sec. Alldrying and firing steps were performed using a Despatch CDF 7210 furnacethat is commonly used for silicon solar manufacturing.

SEM/EDS analysis was used to determine the elemental composition ofvarious regions in polished, cross-sectioned fired multilayer stacks andto study the intercalation process. SEM/EDX was performed with theequipment previously described using two different modes of operation.SEM micrographs were imaged with a Zeiss Gemini Ultra-55 analyticalfield emission SEM using two modes referred to as the SE2 and Inlens.The SE2 mode was operated at 5-10 kV and a working distance of 5-7 mmwith the SE2 secondary electron detector and scanning cycle time of 10seconds. The brightness and contrast were varied between 0 and 50% andbetween 0 and 60%, respectively in order to maximize contrast betweenthe intercalation region and the Al particles. The Inlens mode wasoperated at 1-3 kV and a working distance of 3-7 mm with the InLenssecondary electron detector and scanning cycle time of 10 seconds. Inorder to image the BSF in the Miens mode, the brightness was set to 0%and the contrast set to around 40%.

In one embodiment of the invention, intercalation pates that include10-15 wt % intercalating particles block interdiffusion between preciousmetal (i.e., silver) and metal particles (i.e., aluminum). Intercalationpaste A (shown in Table I) includes 12.5 wt % bismuth particles and 50wt % Ag, resulting in a ratio of intercalating particles to preciousmetal particles of 1:4 by weight. A fired multilayer stack was made asdescribed above. SEM of the fired multilayer stack was performed in SE2mode with the equipment described above at an accelerating voltage of 5kV, a working distance of 7 mm, and 4000 times magnification.

FIG. 6 is a scanning electron microscope cross-section image of theco-fired multilayer stack. There is a modified intercalation layer 630directly on a modified metal particle layer 622. The modifiedintercalation layer 630 includes a bismuth-rich (intercalation phase)sublayer 632 that comprises bismuth oxide and a silver-rich (preciousmetal) sublayer 634. The modified metal particle layer 622 containsaluminum particles 621 and intercalation phase material 623 that hasspread out from the bismuth rich sublayer 632. The intercalation region632 is directly on the aluminum particles 621, at least near interfaceregion 631. The intercalation sublayer 632 seems to preventinterdiffusion of silver from the modified intercalation layer 630 andaluminum from the modified metal particle layer 622 during the co-firingprocess. FIG. 6 is an example of the layered structure described in FIG.4 above. The precious metal sublayer 634 provides a surface (away fromthe modified metal particle layer 622) that is highly solderable. Theintercalation phase material 623 does not penetrate far into themodified metal particle layer 622. The modified metal particle layer 622contains mostly aluminum particles that are weakly sintered together andhave poor mechanical strength after co-firing. There was not enoughbismuth available to penetrate deeply into the metal particle layer 622,and the intercalation sublayer 632 may apply stress to the modifiedmetal particle layer 622, which can mechanically weaken the co-firedmultilayer stack. The peel strength of such a co-fired multilayer stackis less than 0.4 N/mm (Newton per millimeter) with the predominantfailure mechanism between Al particles. Current solar industry standardsrequire peel strengths greater than 1 N/mm to be considered commerciallyviable.

Intercalation paste B (shown in Table I) uses a glass frit as theintercalating particles to achieve a solderable surface. Intercalationpaste B contains 30 wt % bismuth-based glass frit (intercalating)particles and 45 wt % Ag, resulting in a ratio of intercalatingparticles to precious metal particles be 1:1.5 by weight. The glass fritcontains mainly bismuth and has a glass transition temperature of 387°C. and a softening point of 419° C. SEM of the fired multilayer stackwas performed in SE2 mode with the equipment described above at anaccelerating voltage of 5 kV, a working distance of 7 mm, and 4000 timesmagnification. FIG. 7 is a scanning electron microscope cross-sectionimage of such a co-fired multilayer stack, according to an embodiment ofthe invention. A modified metal particle layer 722 contains aluminumparticles 730. During co-firing, the bismuth-based glass frit does notcompletely phase separate from the Ag particles, resulting in a modifiedintercalation layer 750 that has two phases: a precious metal phase 721and a bismuth-based intercalation phase 740 similar to those shown inFIG. 3 above. A surface 750S on the modified intercalation layer 750contains more than 50% precious metal phase 721. The surface 750S issolderable with fluxes commonly used in the solar cell industry (e.g.,Kester 952S, Kester 951 and Alpha NR205). The overall peel strength ofthe fired multilayer stack is less than 0.5 N/mm, which may be due tothe relatively low penetration of the bismuth intercalation phase 740into the modified aluminum particle layer 722. In general, themorphology of the modified intercalation layer can be modified bychanging the intercalating particles composition and loading in theintercalation layer.

Intercalation Pastes Blocking Elemental Interdiffusion and StrengtheningUnderlying Metal Particle Layers

The above examples illustrate two paste formulations engineered to blockinterdiffusion between precious metal (i.e., silver) and metal particles(i.e., aluminum) but whose fired layers lack adequate mechanicalstrength when soldered. Intercalation paste C (shown in Table I)contains 30 wt % bismuth particles and 45 wt % silver particles (i.e.,Ag:Bi intercalation paste), resulting in a ratio of intercalatingparticles to precious metal particles of 1:1.5 by weight. The increasedintercalating particle content in the paste yields a higherconcentration of intercalation material in the modified metal particlelayer and results in a mechanically stronger fired multilayer stack.Intercalation paste C was used as a drop-in replacement for a commercialsilver rear tabbing paste during the fabrication of a BSF,multicrystalline, p-type solar cell. Intercalation paste C may also bereferred to as a silver-on-aluminum (Ag-on-Al), rear tabbing, floatingrear tabbing, or tabbing intercalation paste. A suite ofcharacterization tools was used on the resulting fired multilayer stackin order to assess the IM:M (intercalation metal: metal) ratio, preciousmetal surface coverage, and determine if crystallites formed in theintercalation region.

The influence of the intercalation layer on the metal particle layer isbest demonstrated by first illustrating the morphology of a firedaluminum particle layer on a silicon substrate in the absence of anintercalation layer. FIG. 8 is a scanning electron microscope (SEM)cross-section image in SE2 mode of such a fired aluminum particle layer822 on a silicon substrate 810 taken from a region of a silicon solarcell that does not contain an intercalation layer. The fired aluminumparticle layer 822 is about 20 μm thick and contains aluminum particles821 and a small amount of inorganic binder (i.e., glass frit) 840. AnInLens mode scanning electron micrograph of the same aluminum particlelayer is shown FIG. 9. In InLens mode, the aluminum particle layer 922,aluminum particles 921 and the silicon substrate 910 are clearlyvisible, in addition to a back-surface field region 970 and a solidifiedaluminum-silicon (Al—Si) eutectic layer 980.

The impact of an intercalation layer on producing a modified metalparticle layer after co-firing can be understood with reference to FIG.10. FIG. 10 is an InLens SEM cross-section image of the same siliconsolar cell used in the image shown in FIG. 8, but taken from a regionthat contains a co-fired multilayer stack made using intercalation pasteC (shown in Table I). The co-fired multilayer stack 1000 contains amodified intercalation layer 1030, a modified aluminum particle layer1022, a solidified Al—Si eutectic layer 1080, an aluminum-dopedback-surface field (BSF) region 1070, and a silicon substrate 1010. Inan exemplary embodiment, the BSF in the silicon substrate is dopedp-type to between 10¹⁷ and 10²⁰ atoms per cm .

An SE2 mode scanning electron micrograph of the co-fired multilayerstack of FIG. 10 is shown FIG. 11. While the InLens mode clearly showsthe BSF region, SE2 mode is the preferred mode to image bismuth(intercalation phase) in the modified aluminum particle layer. Theco-fired multilayer stack 1100 contains a modified intercalation layer1130, a modified aluminum particle layer 1122 and a silicon substrate1110. A silver sublayer 1134 and a bismuth intercalation sublayer 1132in the modified intercalation layer 1130 can also be seen. The BSFregion and the solidified Al—Si eutectic layer cannot be seen clearly inthis image. The modified aluminum particle layer 1122 contains a largequantity of bismuth intercalation material 1103, which intercalatedaround aluminum particles 1102 during co-firing. In some instances, thecontrast between the bismuth and silver may not be strong enough toclearly identify the sublayers and degree of bismuth intercalation intothe aluminum particle layer. In such instances,' elemental maps of thecross-sections can be made using SEM/EDX in order to fully determinesilver and bismuth placement in the co-fired multilayer stack.

The amount of intercalating metal (i.e., bismuth) in a modified aluminumparticle layer due to intercalation can be determined by comparing EDXspectra taken from a modified aluminum particle layer region and analuminum particle layer region in the same cross-section sample. It ismost useful if the regions are more than 1μm apart from one another. Away of making this comparison has been described above as an IM:M orBi:(Bi+Al) ratio. Such analysis can be useful in determining whether anintercalation paste was used in the fabrication of a solar cell.Metallization layers in solar cells typically consist of a narrow subsetof metals, which include aluminum, silver, bismuth, lead, and zinc. Incommercial solar cells the aluminum particle layer contains aluminumalmost exclusively.

In one example, intercalating particles in intercalation paste C containbismuth exclusively, and metal particles in the metal particle layer aremostly aluminum. Comparing the ratio of bismuth to bismuth plus aluminum(Bi:(Bi+Al)) in the aluminum particle layer (i.e., which had nointeraction with the intercalation paste) and the modified aluminumparticle layer is a useful metric in determining whether anintercalation paste was incorporated into a solar cell. The EDX spectrafor these two layers was measured for approximately three minutes usingthe equipment described above at an accelerating voltage of 20 kV, and aworking distance of 7 mm. The EDX spectrum for the fired aluminumparticle layer 822 in FIG. 8 was collected from region 898. The EDXspectrum for the modified aluminum particle layer 1122 in FIG. 11 wascollected from region 1199. Elemental quantification was performed onthese spectra using Bruker Quantax Esprit 2.0 software for automaticelemental identification, background subtraction, and peak fitting. TheEDX spectra are shown in FIG. 12. Aluminum and bismuth metal peak areaswere quantified and wt % calculated from the EDX spectra in FIG. 12 forthe two layers and are summarized below in Table II. No significantquantities of any other metals could be identified in the EDX spectra.The aluminum particle layer EDX spectra shown in FIG. 12A yields aBi:(Bi+Al) wt % ratio of 1:244, and the modified aluminum particle layerspectrum shown in FIG. 12B yields a Bi:(Bi+Al) wt % ratio ofapproximately 1:4 as shown in Table II. The Bi:(Bi+Al) wt % ratio in themodified aluminum particle layer 1122 is around 62 times higher than inthe fired aluminum particle layer 822 not in contact with the Ag:Biintercalation layer. In various embodiments, the ratio Bi:(Bi+Al) in afired multilayer stack made is at least 20%, or at least 50%, or atleast 2×, or at least 5×, or at least 10× or at least 50× higher in themodified aluminum particle layer than in the fired aluminum particlelayer.

TABLE II Aluminum Bismuth EDX Quantification and resulting Bi:(Bi + Al)Wt % Ratios Bi:(Bi + Al) Al Bi ratio Aluminum Particle Layer 40.2900.166 1:244  Modified Aluminum Particle Layer 43.641 14.974 1:3.91

Plan view EDX can be used to determine the concentration of elements onthe surface of a rear tabbing layer in a silicon solar cell. In planview, EDX probes regions the surface to depths of about 4 μm or less,making this a useful technique for identifying the degree ofinterdiffusion in a co-fired multilayer stack: a higher precious metalconcentration means there has been less interdiffusion, and a lowerprecious metal concentration means there has been more interdiffusion.FIG. 13 is a plan view EDX spectrum taken from a surface of a reartabbing layer that contains a Ag:Bi intercalation layer, according to anembodiment of the invention. The EDX spectrum was collected using a SEMoperated at an accelerating voltage of 10 kV, 7 mm working distance, and500 times magnification. The dominant peaks between 3.5 and 4 keV andthe smaller peak at 0.3 keV are all identified as silver. The remainingminor peaks in the spectrum are identified as the following: carbon at0.3 keV (convoluted with a minor silver peak); oxygen at 0.52 keV;aluminum at 1.48 keV; and bismuth at 2.4 keV. Element quantification wasperformed automatically using Bruker Quantax Esprit 2.0 software tosubtract the background, identify the elemental peak, and then fit thepeak intensities of the x-ray energies. The normalized weightpercentages of each element are shown in Table III below. The totalsilver coverage on the surface of the rear tabbing layer is 96.3 weightpercent (wt %).

TABLE III Normalized Elemental Weight Percent of the Rear Tabbing LayerSurface Element Normalized Wt % Carbon 0.784 Silver 96.342 Silicon 0.002Aluminum 0.153 Bismuth 1.912 Oxygen 0.807

Intercalation layers that contain silver and bismuth can form severalunique crystal phases when fired on dried aluminum-based metal particlelayers. XRD can be used to distinguish between a fired multilayer stackthat uses bismuth particles in the intercalation layer and a firedmultilayer stack that uses a conventional silver-based tabbing pastewith less than 10 wt % glass frit as the inorganic binder. XRD wasperformed using a Bruker ZXS D8 Discover GADDS x-ray diffractometerequipped with a VÅNTEC-500 area detector and a cobalt x-ray sourceoperated at 35 kV and 40 mA. XRD patterns of fired multilayer stacks onthe rear tabbing layer of a silicon solar cell are shown in FIG. 14.Diffractograms were measured using cobalt Ka wavelength in two 25°frames that were combined for a total window of 25-80° in 2Θ. Each framewas measured for 30 minutes under x-ray irradiation. No backgroundsubtraction was performed on the two diffraction patterns in FIG. 14.Patterns were normalized to unity against the largest peak, and a 0.01background was added to the data to plot in Log(Intensity).

The XRD diffraction patterns show that a fired multilayer stack formedwith an Ag:Bi intercalation layer, or rear tabbing layer in a solarcell, has a different pattern compared to one formed without bismuth.XRD pattern A is from a co-fired multilayer stack on the rear tabbinglayer of a silicon solar cell. The co-fired multilayer stack included amodified intercalation layer formed using an intercalation paste thatcontained approximately 45 wt % silver, 30 wt % Bi, and 25 wt % organicvehicle (as for Paste C in Table I above). Peaks 1410 are identified assilver and peaks 1420 are crystallites of bismuth oxide (Bi₂O₃). XRDpattern B is from a co-fired multilayer stack on the rear tabbing layerof a silicon solar cell formed using a commercially available reartabbing paste that contained less than 10 wt % glass frit as theintercalation layer on an aluminum particle layer. The co-firedmultilayer stack is dark in color, indicating significantsilver-aluminum interdiffusion. Peaks 1450 are identified as asilicon-aluminum eutectic phase. Peaks 1460 are identified as asilver-aluminum alloy phase (i.e., Ag₂Al). Silver peaks 1410 areobserved in pattern A along with a bismuth oxide compound and not inpattern B where silver is observed only as part of a silver-aluminumalloy 1450. This is further evidence that bismuth preventsinterdiffusion in fired multilayer stacks. In one embodiment, a reartabbing layer in a silicon solar cell contains crystallites of bismuthand at least one other element such as silicon, silver, and oxidesthereof, alloys thereof, composites thereof, or other combinationsthereof. In another embodiment, a rear tabbing layer contains bismuthoxide crystallites. In another embodiment, an intercalation regionundergoes multiple phase transformations during firing.

An Intercalation Layer Can Etch Through a Dielectric Layer During Firing

In some device applications, a dielectric layer is deposited onto asubstrate surface before metal layers are deposited in order topassivate the substrate surface and to improve electronic properties.The dielectric layer may also prevent species interdiffusion between thesubstrate and adjacent metal particle layer(s). In some cases, it may behighly desirable to etch through the dielectric layer in order to formcompounds between the substrate and the metal particle layer to improveelectrical conduction between the substrate and the metal particlelayer. Glass frits that contain bismuth and lead are known to penetratethrough a variety of dielectric layers (e.g., silicon nitride) duringco-firing of silicon solar cells. In an exemplary embodiment,intercalation paste D (from Table I above) contains approximately 30 wt% silver, 20 wt % intercalating particles (15 wt % metallic bismuthparticles, 5 wt % high-lead-content glass frit) and 50 wt % organicvehicle. Such an intercalation paste can be especially useful if etchingthrough a dielectric layer is desired.

FIG. 15 shows a schematic cross-section drawing of a multilayer stack1500 that includes a substrate 1510 coated with at least one dielectriclayer 1513 prior to firing, according to an embodiment of the invention.A dried metal particle layer 1520, is on a portion of the dielectriclayer 1513. An intercalation layer 1530, made up of intercalatingparticles and precious metal particles, as described above, is on aportion of the dried metal particle layer 1520. Prior to firing, theprecious metal particles and intercalating particles may behomogeneously distributed within the intercalation layer 1530. Thedielectric layer contains at least one of silicon, aluminum, germanium,hafnium, gallium, oxides thereof, nitrides thereof, composites thereof,and combinations thereof. In one arrangement, the dielectric layer 1513is a 75-nm thick silicon nitride layer. In another embodiment, there isa second dielectric layer (not shown) between the dielectric layer 1513and the substrate 1510. In one arrangement, the second dielectric layeris a 10-nm thick alumina layer directly on the substrate 1510 and thedielectric layer 1513 is a 75-nm thick silicon nitride layer directly onthe alumina layer. Dried metal particle layer 1520 is formed bydepositing metal particle paste on the dielectric layer 1513 andsubsequently drying. In one arrangement, the dried metal particle layer1520 is 20 μm thick and contains aluminum particles. An intercalationlayer 1530 that contains intercalating particles, such as glass frit(s)that contain lead or bismuth, is deposited on the dried metal particlelayer 1520, covering at least a portion of the dried metal particlelayer 1520, and then dried.

FIG. 16 is a schematic cross-section drawing that shows a firedmultilayer stack 1600 (the structure 1500 of FIG. 15 after it has beenfired), according to an embodiment of the invention. A portion ofsubstrate 1610 is coated with at least one dielectric layer 1614. Duringco-firing at least some of the intercalating particles (that includeglass frit(s) as described in reference to FIG. 15) in a modifiedintercalation layer 1630 melt and begin to flow, intercalating into amodified metal particle layer 1622. In one arrangement, material fromglass frit particles in the modified intercalation layer 1630 penetratesinto and through the metal particles in the modified metal particlelayer 1622 and etches into the dielectric layer 1613 (1513 beforefiring), allowing some metal from the modified metal particle layer 1622to interact chemically and electrically with the substrate 1610, formingone or more new compounds 1614. Other intercalating particles (e.g.,bismuth particles) from the modified intercalation layer 1630 may alsointercalate into the modified metal particle layer 1622 and may providestructural support. In one arrangement, as described in more detailabove in reference to FIG. 2, at least a portion of the precious metalparticles and intercalating particles in the modified intercalationlayer 1630 form phases that phase separate from each other. In somearrangements, there are also metal particle regions 1620 (on thedielectric layer 1613) into which little or only trace amounts ofintercalating particle material has penetrated. In an exemplaryembodiment, the intercalating particles are bismuth particles and glassfrit particles, the metal particles are aluminum.

Introducing Variations in the Thickness of a Metal Particle Layer toReduce Buckling

An intercalation layer can cause stress in an underlying modified metalparticle layer during firing, which can cause buckling or wrinkling andtherefore poor layer strength and electrical communication betweenlayers. For example, an intercalation layer may have a differentcoefficient of thermal expansion than an adjacent modified metalparticle layer, causing the layers to expand or contract differentlyduring firing. Another source of stress in an adjacent modified metalparticle layer may be the intercalation of melted intercalating particlematerial between the metal particles. Such stresses can cause a modifiedmetal particle layer and/or a modified intercalation layer to buckle orwrinkle. Buckling or wrinkling can be described as large, periodic ornon-periodic deviations in the thickness of a layer. Often this resultsin delamination between layers. For example, before an intercalationlayer on a dried metal particle layer is fired, the initial thickness ofthe stack that contains the intercalation layer and the dried metalparticle layer is approximately the same everywhere. After co-firing,the thickness of the fired multilayer stack that contains the modifiedintercalation layer and the modified metal particle layer may be by asmuch as three times greater than the initial thickness in some regions.

FIG. 17 is a plan-view optical micrograph of co-fired multilayer stackin which buckling has occurred. A modified intercalation layer 1730 isvisible. The modified intercalation layer 1730 has buckled; some peakregions 1712 are indicated in FIG. 17. An adjacent metal particle layer1720 has not buckled and remains smooth or approximately planar. Eventhough the intercalation layer 1730 has buckled, the mechanicalintegrity of the co-fired multilayer stack remains strong with peelstrengths of more than 1 N/mm. However, the buckling can make itchallenging to make good, robust contact between the modifiedintercalation layer 1730 and tabbing ribbons (not shown) when they aresoldered together. The buckled surface of the modified intercalationlayer 1730 may result in incomplete solder wetting over the extent ofintercalation layer 1730, which can lower peel strength and solder jointreliability. It may be useful to reduce or eliminate buckling in theco-fired multilayer stack to ensure successful soldering to tabbingribbons.

Variable thicknesses can be incorporated into fired multilayer stacks tosignificantly reduce buckling and/or wrinkling of layers. When one ormore layers has variable thicknesses, a non-planar interface betweensuch layers can result. An indication of variable thickness is anon-planar interface between layers in a fired multilayer film stack.Variable thickness can be created by patterning a portion of a firstlayer and subsequently printing a second layer directly on the patternedportion of the first layer to create a non-planar interface between thetwo layers. In one arrangement, a layer has variable thickness as theresult of having been printed using a patterned screen. After firing,the thicknesses of individual layers may be reduced, but firing does notcause layers with variable thickness to become layers with uniformthickness. Variable thickness in a layer can be measured and quantifiedusing cross-sectional SEM and surface topology techniques both beforeand after firing. In various embodiments, a layer can be described ashaving variable thickness when it has thickness variations that are atleast 20% greater than or at least 20% less than the average thicknessof the layer as measured within a 1×1 mm area.

FIG. 18 is a screen that can be used during deposition of metal particlepaste to achieve variable thickness in a dried metal particle layeraccording to an embodiment of the invention. The screen 1800 has an openmesh 1810, and some patterned regions 1820. The patterned regions 1820contain closed areas 1821 and open areas 1822. When the screen 1800 isused during printing of a wet metal particle layer, paste flows throughthe openings 1822 and the open mesh 1810 and is blocked by the closedareas 1821, which causes the deposited wet metal particle layer to havevariable thickness. In one embodiment, the wet metal particle layer issubsequently dried to form a variable-thickness dried metal particlelayer, and an intercalation paste is deposited directly on thevariable-thickness dried metal particle layer.

There are several factors that can affect variable thickness in a driedmetal particle layer, such as mesh count, wire diameter and shape, wireangle relative to the frame, emulsion thickness and screen design. Themesh size and wire diameter determine the minimum pattern shapes andopenings that can be printed. Thickness variations in a dried metalparticle layer are also affected by the rheology of the metal particlepaste, which affects layer slumping. Pastes can be designed with highviscosities and thixotropies to control exactly where they are depositedon a substrate. It is also possible to change the magnitude of thethickness variations in the metal particle layer by adjusting theemulsion thickness of the screen. Screens may be designed to ensure acontinuous dried metal particle layer on a substrate surface withvariable layer thicknesses overall or only in particular regions. In anexemplary embodiment, a metal particle paste is printed using a 230-meshscreen with an emulsion thickness of 5 μm. In one arrangement, thepattern 1820 has a series of 100 μm by 3 mm open areas 1822 adjacent to100 μm by 3 mm closed areas 1821. There is no restriction on patterntype, periodicity (or lack thereof), or size. Many patterns can resultin variable thickness and the patterns may be adjusted for variousprinting conditions and paste formulations.

FIG. 19 is a schematic cross-section drawing of a dried metal particlelayer with variable thickness deposited onto a substrate 1910 using thescreen 1800 of FIG. 18, according to an embodiment of the invention. Adried metal particle layer 1920 outside region 1925 was formed bydepositing metal particle paste through open mesh areas 1810 of thescreen 1800, and then drying the metal particle paste. Avariable-thickness dried metal particle layer 1922 in region 1925 wasdeposited through a masked region 1820 of the screen 1800 and hasvariable thickness. An intercalation paste was subsequently printeddirectly on the variable-thickness dried metal particle layer 1922 inregion 1925 and dried to form an intercalation layer 1930.

FIG. 20 is a schematic cross-section drawing of the structure of FIG. 19after it has been co-fired, according to an embodiment of the invention.As described above, co-firing causes material from the intercalationlayer 1930 (FIG. 19) to intercalate into the underlyingvariable-thickness dried metal particle layer 1922 (FIG. 19),transforming the variable thickness metal particle layer 1922 into avariable-thickness modified metal particle layer 2022 and theintercalation layer 1930 into a modified intercalation layer 2030. Inone arrangement, the modified metal particle layer 2022 has patternedthickness variations including, but not limited to, periodic bumps,ridges, edges and other feature shapes. It should be noted that thethickness of the modified intercalation layer 1930 is often uniform, andthe non-planar interface between the modified intercalation and themodified metal particle layer (due to its variable thickness) can beinferred by measuring variations in the total layer thickness of themultilayer stack.

FIG. 21 is a plan-view optical micrograph of a co-fired multilayer stackin which metal particle paste was printed with variable thickness (insome regions) using a screen such as that shown in FIG. 18. Anintercalation layer was printed directly on the variable thicknessregion of the metal particle layer, and the multilayer stack wasco-fired to form a modified intercalation layer 2121 on the top surface,bordered on either side by an approximately planar metal particle layer2120. The metal particle layer 2120 has a planar top surface. Thesurface of the modified intercalation layer 2121 is non-planar with apattern that reflects the variations in thickness in the underlyingmodified metal particle layer. The surface of the modified intercalationlayer 2121 does not show signs of buckling or wrinkling, as were seenclearly in the modified intercalation layer 1730 in FIG. 17. In oneembodiment of the invention, a portion of the co-fired multilayer stackhas variable thickness.

A useful metric to describe variable thickness is to compare peakthicknesses and valley thicknesses to average layer thickness. In anylayer, there can be some unintentional thickness variation, but suchvariations are typically less than 20% of the average layer thickness. Alayer can be considered to be planar (have uniform thickness) if itsthickness varies by less than 20% of the average layer thickness. Bycareful design of screens for printing metal particle pastes, it ispossible to create layers with variable thicknesses that have thicknessvariations that are at least 20% greater than or at least 20% less thanthe average thickness of the layer as measured within a 1×1 mm area.

Variable thickness in fired multilayer stacks can be measured from SEMimages of polished cross-section samples. FIG. 22 is a cross-section SEMimage of a portion of a fired multilayer stack 2210 that has variablethickness, according to an embodiment of the invention. Cross-sectionsamples were prepared and imaged using the methods described above. Thefired multilayer stack 2210 includes a modified intercalation layer2211, a modified aluminum particle layer 2212, and a silicon substrate2213. Two interfaces on either side of the modified aluminum particlelayer 2212 are identified in the image: interface 2218 between thesilicon substrate 2213 and the modified aluminum particle layer 2212,and interface 2217 between the modified aluminum particle layer 2212 andthe modified intercalation layer 2211. Interface 2216 is a solderablesurface. For comparison, FIG. 23 shows a silicon substrate 2322 that hasa planar aluminum particle film 2321 that does not have variablethickness.

The average thickness of the modified aluminum particle layer 2212 inFIG. 22 is calculated by averaging thicknesses measurements. Thethickness between the two interfaces 2217 and 2218 in FIG. 22 wasmeasured at regular intervals (e.g., 10 microns) across the sample.Thicknesses were also measured at local maximums and local minimums.Software such as ImageJ 1.50 a can be used to obtain average thickness,and minimum and maximum thicknesses. The peaks and valleys seen in asingle cross-section sample may not be representative of an entire firedmultilayer stack. Therefore, it is useful to make such measurements overseveral cross-section samples in order to ensure that very many peaksand valleys are measured. These are methods that would be known to aperson with ordinary skill in the art.

For the sample shown in FIG. 22, the modified aluminum particle layer2212 has an average thickness of 11.3 μm, a peak thickness of 18.4 μm,and a valley thickness of 5.2 μm. The peak thickness is 64% greater thanthe average thickness and the valley is 54% less than the averagethickness. In various embodiments, a layer with variable thickness has apeak thickness that is at least 20%, at least 30%, at least 40%, or atleast 50% greater than the average layer thickness. In variousembodiments, a layer with variable thickness has a valley thickness thatis at least 20%, at least 30%, at least 40%, or at least 50% less thanthe average layer thickness.

When the modified intercalation layer 2211 is continuous andapproximately uniform in thickness, solderable surface 2216 of themodified intercalation layer 2211 is approximately parallel to theinterface 2217. In one embodiment of the invention, all measurementsdescribed above for the modified aluminum particle layer 2212 can bemade for the combined thicknesses of the modified aluminum particlelayer 2212 and the modified intercalation layer 2211 between thesolderable surface 2216 and the interface 2217. Comparisons of thethickness measurements for the two combined layers are goodapproximations for comparisons of the thickness measurements for themodified aluminum particle layer 2212 alone. For the combined layers inFIG. 22, the peak thickness is 44% greater than the average overallthickness of 13.2 μm, and the valley thickness is 43% less than theaverage overall thickness. This alternative method may systematicallyunder-measure the thickness variation in a fired multilayer stack.

For some applications, it may be desirable for only a portion of a firedmultilayer stack to have variable thickness. For example, an aluminumparticle layer on the back side of a silicon solar cell is typicallyplanar. It may be useful to introduce variable thickness in rear tabbinglayer portions (which include modified intercalation layers) on the backside of such a cell. Comparing thickness variation in a portion of therear tabbing layer with thickness variation in a portion of thesurrounding aluminum particle layer could be used to determine if layerswith variable thicknesses were used on the back side of a solar cell.

Another useful metric to determine variable thickness in a firedmultilayer film stack is average peak-to-valley height, which is thedifference between the average of local maximums and the average oflocal minimums. In cross-section SEM images there is no guarantee thatlocal maximums and local minimums in the image, so surface topographicmetrology methods such as profilometery, coherent scanninginterferometry, and focus variation microscopy are more useful. Anexample of a profilometer is a Bruker or Veeco Dektak 150 or equivalent.Coherent scanning interferometry can be performed using an Olympus LEXTOLS4000 3D Laser Measuring Microscope. An accompanying software withthese methods can automatically calculate average peak-to-valleydifference.

In an example embodiment, profilometry is used to determine, within thesame sample, the average peak-to-valley heights both for a firedmultilayered stack that has variable thickness and for an aluminumparticle layer that has uniform thickness. A Veeco Dektak 150 was usedto measure the surfaces in a 1×1 mm area with a 12.5 mm radius probe toproduce 3D topological surface maps. FIG. 24 is a 3D surface topologymap of the fired multilayered stack with variable thickness, and FIG. 25and is a 3D surface topology map of the (adjacent) aluminum particlelayer with uniform thickness. The lightest regions in the figuresindicate local maximums and the darkest regions indicate local minimums.FIG. 24 shows thickness variations (from −20.2 μm to 15.9 μm), whichwould be expected for fired multilayer stack that included a variablethickness modified metal particle layer. FIG. 25 shows thicknessvariations (from −4.9 μm to 5.5 μm), which would be expected for analuminum particle layer that has uniform thickness. Averagepeak-to-valley heights were calculated using the program Veeco Visionv4.20 which automatically identifies and averages local maximums andminimums and then subtract the difference. The average peak-to-valleyheight for the fired multilayer stack in FIG. 24 is 35.54 μm and for thealuminum layer in FIG. 25 is 9.51 μm. In various embodiments, a layerhas variable thickness when the average peak-to-valley height is greaterthan 10 μm, greater than 12 μm, or greater than 15 μm, and a layer hasuniform thickness when the average peak-to-valley height is less than 10μm, less than 12 μm, or less than 15 μm.

In one embodiment of the invention, when the modified intercalationlayer of a co-fired variable-thickness multilayer stack, such as the oneshown in FIG. 20, is soldered to a tabbing ribbon, it has a peelstrength that is twice the peel strength of a fired multilayer stackthat does not have variable thickness. In one arrangement, the modifiedintercalation layer on the surface of such a variable-thickness firedmultilayer stack is soldered to a tin-based tabbing ribbon, and theyhave a peel strength greater than 1.5 N/mm, or greater than 2 N/mm, orgreater than 3 N/mm. The thickness variations may be optimized toprovide a continuous metal particle layer and back-surface field on thesubstrate for a silicon solar cell. The thickness variations may beoptimized so that the contact resistance for such co-firedvariable-thickness multilayer stacks is equivalent to or lower than thecontact resistance for approximately planar co-fired multilayer stacks.In an exemplary embodiment, when using an intercalation paste to etchthrough a dielectric layer, thickness variations in dry and modifiedmetal particle layers include regions that are less than 20 μm, 10 μm, 5μm, or 2 μm thick.

The variable thickness layer(s) described above, can be used ascomponent(s) in any of the fired multilayer stacks described herein.Variable thickness layer(s), such as variable thickness dried andmodified metal particle layers, can be used on any silicon solar cellsto reduce buckling of the rear tabbing layer.

Intercalation Pastes as Drop-In Replacements in Silicon Solar Cells

In one embodiment, intercalation paste that contains 45 wt % preciousmetal particles, 30 wt % intercalating particles, and 25 wt % organicvehicle (Paste C in Table I above) can be used as a drop in replacementto form a rear tabbing layer in a silicon solar cell. Fabrication of p-njunction silicon solar cells is well known in the art. Goodrich et al.provide a full process flow to fabricate a back surface field siliconsolar cell, which is referred to as “standard c-Si solar cell.” SeeGoodrich et al. “A wafer-based monocrystalline silicon photovoltaicsroad map: Utilizing known technology improvement opportunities forfurther reductions in manufacturing costs”, Solar Energy Materials andSolar Cells (2013) pp. 110-135, which is incorporated by referenceherein. In one embodiment, a method for fabricating a solar cellelectrode includes the steps of providing a silicon wafer with a portionof the front surface covered in at least one dielectric layer, applyingan aluminum particle layer onto the back surface of the silicon wafer,drying the aluminum particle layer, applying an intercalation paste(rear tabbing) layer onto a portion of the aluminum particle layer,drying the intercalation paste layer, applying a plurality of fine gridlines and at least one front busbar layer onto the dielectric layer onthe front surface of the silicon wafer, drying and co-firing the siliconwafer. Methods such as screen printing, gravure printing, spraydeposition, slot coating, 3D printing and/or inkjet printing can be usedto apply the various layers. As an example, Ekra or Baccini screenprinters can be used to deposit the aluminum particle layer,intercalation paste layer and front side grid lines and busbar layers.In another embodiment, the solar cell has at least one dielectric layercovering at least a portion of the rear surface of the silicon wafer.For a PERC (passivated emitter rear cell) architecture, two dielectriclayers (i.e., alumina and silicon nitride) are applied to the rear sideof the silicon solar cell prior to the application of the aluminumparticle layer. Drying various layers may be done in a belt furnace attemperatures between 150° C. and 300° C. for 30 seconds to 15 minutes.In one arrangement, a Despatch CDF 7210 belt furnace is used to dry andco-fire the silicon solar cells that contain fired multilayer stacks asdescribed herein. In one arrangement, co-firing is done using a rapidheating technique and heating to a temperature greater than 760° C. forbetween 0.5 and 3 seconds in air, which are common temperature profilesfor aluminum back-surface field silicon solar cells. The temperatureprofile of a wafer is often calibrated using a DataPaq® system with athermocouple attached to the bare wafer.

FIG. 26 is a schematic drawing that shows the front (or illuminated)side of a silicon solar cell 2600. The silicon solar cell 2600 has asilicon wafer 2610 with at least one dielectric layer (not shown), ontop of which there are fine grid lines 2620 and front side busbars lines2630. In one embodiment, the dielectric layer on the front side of thesilicon wafer contains at least one material selected from the groupconsisting of silicon, nitrogen, aluminum, oxygen, germanium, hafnium,gallium, composites, and combinations thereof. In another embodiment thedielectric layer on the front side of the silicon wafer is siliconnitride and is less than 200 nm thick. Commercially-available front-sidesilver metallization pastes that are known in the art can be used toform the fine grid lines 2620 and the front busbar lines 2630. It shouldbe noted that the front side silver layers (i.e., fine grid lines 2620and front busbar lines 2630 made from silver metallization pastes) mayetch through the dielectric layer during co-firing and make directcontact with the silicon wafer 2610. In one embodiment, the siliconwafer 2610 is monocrystalline and doped either n-type or p-type. Inanother embodiment, the silicon wafer 2610 is multicrystalline and dopedeither n-type or p-type. In an exemplary embodiment, the substrate is amulticrystalline, p-type silicon wafer with an n-type emitter.

FIG. 27 is a schematic drawing that shows the rear side of a siliconsolar cell 2700. The rear side is coated with an aluminum particle layer2730 and has rear side tabbing layers 2740 distributed over a siliconwafer 2710. In one embodiment, the dielectric layer on the rear sidecomprises at least one material selected from the group consisting ofsilicon, nitrogen, aluminum, oxygen, germanium, hafnium, gallium,composites, and combinations thereof on the front surface of the siliconwafer. In another exemplary embodiment the dielectric layer on the frontsurface of the silicon wafer is silicon nitride and less than 200 nmthick. In one embodiment there is no dielectric layer on the rear sideof the silicon wafer. Commercially available aluminum paste that isknown in the art can be printed on at least 85%, or at least 90% or atleast 95%, or at least 97% of the total surface area of the back of thesilicon wafer before firing, which can be described as full Al coverage.The aluminum particle layer (after co-firing) 2730 has an averagethickness between 20 and 30 μm. In various embodiments, the aluminumparticle layer 2730 has a porosity between 3 and 20%, between 10 and18%, or any range subsumed therein. For conventional BSF (back surfacefield) solar cell architectures the rear tabbing layer is applieddirectly to the silicon wafer. However, to improve power conversionefficiency of the solar cell, it can be useful to print a rear tabbinglayer on the aluminum particle layer. In one embodiment, anintercalation layer is applied directly onto a portion of the driedaluminum particle layer to form a rear tabbing layer 2740. FIG. 27 showsone possible pattern for a rear tabbing layer 2740. The intercalationlayer and underlying aluminum particle layer are eventually co-fired toform a fired multilayer stack as described herein. In variousarrangements, the modified intercalation layer (or rear tabbing layer)2740 has an average thickness between 1 μm and 20 μm, or between 2 μmand 10 μm, or between 2.5 μm and 8 μm.

A variable thickness metal (aluminum) particle layer, previouslydescribed above, can be used on the back side of silicon solar cells toreduce buckling of the rear tabbing layer and to improve adhesion andelectrical contact. In one embodiment of the invention, a portion of therear tabbing layer has variable thickness. In another embodiment of theinvention, a portion of the modified aluminum particle layer hasvariable thickness. In one arrangement, the rear tabbing layer on thesurface of such a variable-thickness modified aluminum particle layer issoldered to a tin-based tabbing ribbon, resulting in a peel strengthgreater than 0.7 N/mm, greater than 1.5 N/mm, greater than 2 N/mm, orgreater than 3 N/mm. The thickness variations can be optimized toprovide a continuous metal particle layer and back-surface field on thesubstrate for a silicon solar cell. In another embodiment, in the reartabbing layer regions, a portion of the combined layer (modifiedaluminum particle layer and rear tabbing layer) in that region has athickness that is at least 20%, 30%, or 40% greater than the averagecombined layer thickness measured over a 1×1 mm area. In anotherembodiment, in the rear tabbing layer regions, a portion of the combinedlayer (modified aluminum particle layer and rear tabbing layer) in thatregion has a thickness that is at least 20%, 30%, or 40% less than theaverage combined layer thickness as measured over a 1×1 mm area.

In one embodiment of the invention, solar cells that include any of thefired multilayer stacks discussed herein can be incorporated into solarmodules. There are many possible solar module designs in which suchsolar cells be used, as would be known to a person with ordinary skillin the art. The number of solar cells in a module is not intended to belimited. Typically, 60 or 72 solar cells are incorporated intocommercially available modules, but it is possible to incorporate moreor less depending on the application (i.e., consumer electronics,residential, commercial, utility, etc.). Modules typically containbypass diodes (not shown), junction boxes (not shown) and a supportingframe (not shown) that do not directly contact the solar cell. Bypassdiodes and the junction box may also be considered part of cellinterconnects.

FIG. 28 is a schematic cross-section drawing that shows a portion of asolar cell module, according to an embodiment of the invention. Thesolar cell module contains at least one silicon solar cell 2840. A frontside 2840F of the silicon solar cell 2840, is attached to a firsttabbing ribbon 2832 (which runs into and out of the page), over whichthere is a front encapsulation layer 2820 and a front sheet 2810. Abackside 2840B of the silicon solar cell 2840, is attached to a secondtabbing ribbon 2834, over which there is a rear encapsulation layer 2850and a back sheet 2860. The tabbing ribbons 2832, 2834 contact adjacentsolar cells electrically through soldered connections to a front side(i.e., front busbar on the front side) of one cell and to a back side(i.e., rear tabbing layer on the back side) of an adjacent solar cell.Large numbers of solar cells in a solar module can be electricallycoupled together using tabbing ribbons as cell interconnects.

Typical cell interconnects include metal tabbing ribbons that aresoldered onto solar cells and metal bus ribbons that connect tabbingribbons. In one embodiment of the invention, a tabbing ribbon is a metalribbon that has a solder coating. Such a solder-coated tabbing ribbonmay have a thickness range of 20 to 1000 μm, 100 to 500 μm, 50 to 300 μmor any range subsumed therein. The width of the solder-coated tabbingribbon may be between 0.1 and 10 mm, between 0.2 and 1.5 mm, or anyrange subsumed therein. The length of the tabbing ribbon is determinedby application, design, and substrate dimensions. The solder coating mayhave a thickness between 0.5 and 100 μm, between 10 and 50 μm, or anyrange subsumed therein. The solder coating may contain tin, lead,silver, bismuth, copper, zinc, antimony, manganese, indium, or alloys,composites, or other combinations thereof. The metal tabbing ribbon mayhave a thickness between 1 μm and 1000 μm, between 50 and 500 μm,between 75 and 200 μm, or any range subsumed therein. The metal tabbingribbon may contain copper, aluminum, silver, gold, carbon, tungsten,zinc, iron, tin or alloys, composites, or other combinations thereof.The width of the metal tabbing ribbon may be between 0.1 and 10 mm,between 0.2 and 1.5 mm, or any range subsumed therein. In oneembodiment, a tabbing ribbon is a copper ribbon that is 200 μm-thick and1 mm wide and is coated on all sides with a 20 μm thick tin:lead (60:40wt %) solder coating.

The front sheet 2810 in FIG. 28 provides some mechanical support to themodule and has good optical transmission properties over the portion ofthe solar spectrum the solar cell 2840 is designed to absorb. Solarmodules are positioned so that the front sheet 2810 faces a source ofillumination, such as sunlight 2860. The front sheet 2810 is typicallymade of low-iron content soda-lime glass. The front encapsulant layer2820 and the back encapsulant layer 2850 protect the solar cell 2840from electrical, chemical and physical stressors during operation.Encapsulants are typically in the form of polymeric sheets. Examples ofmaterials that can be used as encapsulants include, but are not limitedto, ethylene vinyl acetate (EVA), poly-ethylene-co-methacrylic acid(ionomer), polyvinyl butyral (PVB), thermoplastic urethane (TPU),poly-α-olefin, poly-dimethylsiloxan (PDMS), other polysiloxanes (i.e.,silicone), and combinations thereof.

The back sheet 2860 provides protection to the solar cell 2840 from therear side and may or may not be optically transparent. Solar modules arepositioned so that the back sheet 2860 faces away from a source ofillumination, such as sunlight 2860. A back sheet 2860 may be amultilayer structure made of three polymeric films. DuPont™ Tedlar®polyvinyl fluoride (PVF) films are typically used in the back sheet.Fluoropolymers and polyethylene terephthalates (PET) can also be used inthe back sheet. A glass sheet may also be used as the back sheet, whichcan aid in providing structural support to the solar module. Asupporting frame (not shown) may also be used to improve structuresupport; supporting frames are typically made of aluminum.

In one embodiment of the invention, a method for forming a solar cellmodule is provided. Solder tabs are applied to individual solar cells(that contain any of the fired multilayer stacks described herein)either manually or by using an automated tabbing or stringing machine.Then the individual cells are electrically connected in series bydirectly soldering them to a tabbing ribbon. The resulting structure isreferred to as a “cell string”. Often multiple cell strings are arrangedonto a front encapsulation layer that has been applied to a front sheet.These multiple cell strings are connected to one another using busribbons to create an electrical circuit. The bus ribbons are wider thanthe tabbing ribbons used in the cell strings. When the electricalcircuit between all cell strings is complete, the rear encapsulationmaterial is applied to the back of the connected cell strings and theback sheet is placed on the rear encapsulation material. The assembly isthen sealed using a vacuum lamination process and heated (typicallybelow 200° C.) to polymerize the encapsulating material. A frame istypically attached around the front sheet to provide structural support.Finally, a junction box is connected to the cell interconnects and isattached to the solar module. Bypass diodes may be in the junction boxor may be attached inside the module during cell interconnectionprocessing.

In one embodiment of the invention, a method of forming a solar moduleincludes: a) providing at least one solar cell that has a front surfaceand a back surface; wherein, the back surface comprises a firedmultilayer stack, b) soldering a portion of a tabbing ribbon onto aportion of a rear tabbing layer and front busbar layer creating a cellstring, c) optionally, soldering tabbing ribbons to bus ribbons tocomplete the electrical circuit, d) arranging cell strings onto a frontencapsulation layer that has been applied to a front sheet, e) applyinga rear encapsulation layer to the cell strings and attaching a backsheet to the rear encapsulation layer to form a module assembly, f)laminating the module assembly, g) electrically connecting andphysically attaching a junction box.

It is possible to disassemble a solar module to determine if a firedmultilayer stack, as described herein, has been incorporated using thefollowing steps. Remove the back sheet and rear encapsulant to exposethe tabbed, rear surface of the solar cells. Apply a fast curing epoxyon a tabbing ribbon and surrounding rear surface of the solar cell.Remove the cell from the module after the epoxy has cured and use adiamond saw to cut a section of tabbing ribbon/solar cell. Use an ionmill previously described to polish the cross-section, and performSEM/EDX to determine if the structure is as described in the embodimentsof the invention. FIG. 29 is a polished cross-section SEM image of aback (non-illuminated) side of a solar cell. The sample is from a solarcell (that includes a novel fired multilayer stack) which had beenincorporated into a solar module and was then removed as describedabove. The image shows a metal tabbing ribbon 2932 and its soldercoating 2931, which are soldered to a fired multilayer stack 2902. Theconstituent layers of the fired multilayer stack 2902 are clearlyvisible. Just below the solder coating 2931 is a modified intercalationlayer 2945, a modified metal particle layer 2944, and a siliconsubstrate 2941. The layers identified in the figure can be more easilyidentified using EDX.

Other PV Cell Architectures

Intercalation pastes can be used to create a variety of fired multilayerstacks that can be used as metallization layers on the front and backside of many different solar cell architectures. Intercalation pastesand fired multilayer stacks, as disclosed herein, can be used in solarcell architectures that include, but are not limited to, BSF siliconsolar cells, passivated emitter and rear contact (PERC) solar cells, andbifacial and interdigitated back contact solar cells.

PERC solar cell architecture improves upon BSF solar cell architectureby reducing back contact surface recombination through the use of adielectric barrier between the silicon substrate and the back contact.In a PERC cell, a portion of the back side (i.e., non-illuminated) of asilicon wafer is passivated with at least one dielectric layer to reduceelectrical current carrier recombination. The novel fired multilayerstacks disclosed herein can be used in PERC solar cells. In oneembodiment, a dielectric layer on the back side of a silicon wafercontains at least one of silicon, nitrogen, aluminum, oxygen, germanium,hafnium, gallium, composites, or combinations thereof. In anotherembodiment, a dielectric layer on the back side of a silicon waferincludes a 10 nm thick alumina layer on the silicon surface and a 75 nmthick silicon nitride layer on the alumina layer. Commonly-used aluminumpastes (e.g., Monocrystal EFX-39, EFX-85) designed for PERC cells do notpenetrate through the dielectric layer(s). In order for an aluminumparticle layer to chemically interact and make ohmic contact to silicon,small some regions of the dielectric layer are removed via laserablation prior to deposition of the aluminum particle layer.

PERL (passivated emitter with rear locally diffused) and PERT(passivated emitter, rear totally diffused) are two variations of PERCcell architecture that further improve device performance. Both of thesevariations rely on doping the rear part of the silicon substrate tofurther suppress recombination at the rear contact, which serves a roleanalogous to the back surface field in BSF cells. In a PERL cell, thebackside of the silicon substrate is doped around the opening in thedielectric that makes contact with the rear aluminum layer. Doping isusually achieved by diffusing a dopant through the dielectric openingsusing a boron compound or aluminum from the aluminum particles that makeup the rear contact, similar to the BSF fabrication process. A PERT cellis similar PERL, but the entire silicon in contact with the reardielectric layer is doped in addition to the silicon adjacent to thedielectric openings that contact the rear contact.

In one embodiment, an intercalation paste, which contains intercalatingparticles that do not etch through dielectric layer(s) is used as therear tabbing layer on PERC, PERL, or PERT cells. The “non-etching”intercalation paste is used both to provide a solderable silver surfaceand to mechanically strengthen the underlying (modified) aluminumparticle layer. The resulting fired multilayer stack contains a siliconwafer, which is covered with at least one dielectric layer, a modifiedaluminum particle layer and a modified intercalation layer; for PERL orPERT, the silicon is doped only at the dielectric openings or alsoacross the dielectric interface, respectively. Using non-etchingintercalation pastes can further reduce etching of the dielectriclayer(s) and reduce surface recombination. For example, busbar pastesthat are conventionally used for the rear tabbing layer in PERC cellsare printed directly onto the dielectric layer and partially etchthrough the dielectric layer, which increases the surface recombination,during co-firing.

For cells that use rear dielectric layers (i.e., PERC, PERL, PERT),intercalation pastes can be modified to etch through the dielectriclayers and assist in the diffused doping of silicon regions at thedielectric openings, according to an embodiment of the invention. The“etching” intercalation paste (e.g., intercalation paste D in Table I)is used to provide a solderable silver surface, mechanically strengthenthe underlying (modified) aluminum particle layer, and etch through thedielectric layer, exposing the silicon surface to the aluminumparticles, which can lead to aluminum doping of the exposed silicon. Theresulting fired multilayer stack contains the silicon wafer, a modifiedaluminum particle layer and a modified intercalation layer. The firedmultilayer stack may further include an Al-doped region near the siliconsurface (similar to the back-surface field in BSF cells) and a solidsilicon-aluminum eutectic layer at the interface between the siliconwafer and the modified aluminum particle layer. Using intercalationpastes to etch through the dielectric layer(s) has several advantages.First, it is an inexpensive alternative to a laser ablation step, whichhas proven to be costly and unreliable in the past. Second, laserablation can often remove tens to hundreds of microns of siliconsubstrate material and can result in large void formation between thesilicon substrate and the aluminum particle layer when the wafer isco-fired. The etching intercalation paste does not cause changes in thewafer surface prior to co-firing, which results in better bondformation, reduced void formation, and better reproducibility than whenlaser ablation is used.

Intercalation pastes can be used to provide a solderable surface forcell architectures that depend on aluminum particle layers to make ohmiccontact to p-type silicon, according to an embodiment of the invention.Examples of these architectures include interdigitated back contactsolar cells, n-type BSF cell architectures and bifacial solar cells. Inone embodiment, intercalation paste C (from Table I) is applied to an Allayer on interdigitated back contact solar architecture, such as a Zebracell. For n-type BSF architectures, which have obtained 20% powerconversion efficiencies for fully Al covered cells, intercalation pastescan replace conventional rear tabbing Ag pastes that directly contactsilicon, thereby reducing the V_(oc) of the solar cell. In severaln-type wafer based solar cell architectures, the intercalation paste maybe used on the front side (i.e., illuminated side). Intercalation pastescan also be used in conjunction with Al pastes to reduce the cost ofbifacial solar cells. Currently bifacial solar cell architectures use Agpastes that contain a small quantity of aluminum (i.e., less than 5 wt %Al) to make ohmic contact to p-type silicon layer. The current bifacialarchitecture uses almost twice the amount of silver as the BSFarchitecture, which can be cost prohibitive. It can be useful to usepure aluminum pastes in bifacial architectures, but Al is notsolderable. Intercalation paste containing silver (e.g., Paste C inTable I) can be printed on Al pastes in the bifacial design and provideboth mechanical stability and a solderable surface while reducing theamount of Ag used.

Materials Properties of Fired Multilayer Stacks and Impact on SiliconSolar Cells

Material properties of interest in fired multilayer stacks for use insolar cells and other electronic devices include solderability, peelstrength, and contact resistance.

Solderability is the ability to form a strong physical bond between twometal surfaces by the flow of a molten metal solder between them attemperatures below 400° C. Soldering on the modified intercalation layerof a fired multilayer stack may be performed after heating in air toover 650° C. Soldering involves the use of flux, which is any chemicalagent that cleans or etches one or both of the surfaces prior to reflowof the molten solder. Solder fluxes typically used for solar cells,designated as either RMA (e.g., Kester® 186) or R (e.g., Kester® 952),are deposited on the tabbing ribbon and dried at 70° C. These fluxes arenot effective at etching many metal oxides such as alumina (Al₂O₃),which forms on aluminum particles when fired in air.

Peel strength is a metric of solder joint strength and an indicator ofreliability for integrated circuit, light emitting diode and solar cellapplications. A solder coated metal ribbon, which is between 0.8 and 20mm wide and 100-300 um thick can be dipped into flux and dried. It isput on the modified intercalation layer and can be soldered using asolder iron at a temperature between 200° and 400° C. The peel strengthis the force required to peel a soldered ribbon, at a 180° angle fromthe soldering direction, divided by the width of the soldered ribbon,for a given peel rate. The solder joints formed during the solderingprocess have a mean peel strength that is greater than 1 N/mm (e.g., a 2mm tabbing ribbon would require a peel force of greater than 2N todislodge the tabbing ribbon) at 1 mm/sec. Solar cells are electricallyconnected by tabbing ribbons, which are soldered to the front busbars ofone cell and the rear tabbing layers of an adjacent cell. It is commonfor the peel strength to be between 1.5 and 4 N/mm on contacts to thetabbing ribbons in commercially available solar cells. When using afired multilayer stack as the rear tabbing layer, a primary failure modecan be near the Al—Si interface, which can be determined using plan viewSEM/EDX. In an exemplary embodiment, the peel strength is greater than1N/mm when the silver rich sublayer layer (of the modified intercalationlayer) is soldered with a tin based tabbing ribbon.

Meier et al. describe how to use a four-point probe electricalmeasurement to determine the resistivity of each metallization layer ona completed solar cell. See Meier et al. “Determining components ofseries resistance from measurements on a finished cell”, IEEE (2006) pp.2615, which is included by reference herein. The bulk resistance of ametallization layer is directly related to the bulk resistance of thematerial from which it is made. In one embodiment of the invention, thebulk resistance of pure Ag is 1.5×10⁻⁸ Ω-m; pure Ag metallization layersused on industrial solar cells have a bulk resistance that is 1.5 timesto 5 times higher than the bulk resistance of pure Ag. The bulkresistance is important for fine grid lines, which must transportcurrent over a relatively long (i.e., more than 1 cm) length. Theresistance of the front busbars and rear tabbing layers are lessimportant when the cells are tabbed in a module.

In most integrated circuits, LED and solar cell architectures,electrical current flows from a metal particle layer through a modifiedmetal particle layer and into a modified intercalation layer. For firedmultilayer stacks, the contact resistance between these three layers canplay an important role in device performance. The contact resistancebetween these layers in the fired multilayer stack can be measured byusing the transmission line measurement (TLM) (Reference: Meier et al.“Cu Backside Busbar Tape: Eliminating Ag and Enabling Full Al coveragein Crystalline Silicon Solar Cells and Modules”, IEEE PVSC (2015) pp.1-6). The TLM is plotted as resistance versus distance betweenelectrodes. TLM was used specifically to measure the contactresistance 1) between a metal particle layer and a modified metalparticle layer and 2) between a modified metal particle layer and amodified intercalation layer. The contact resistance of a firedmultilayer stack is the sum of contact resistances 1) and 2) above. Thecontact resistance of a fired multilayer stack is half of they-intercept value of a linear fit of the resistance vs. distancemeasurements. The electrical resistance between busbars was measuredusing a Keithley 2410 Sourcemeter in a four-point probe setup thatsourced current between −0.5A and +0.5A and measured the voltage. Invarious embodiments, the contact resistance of a fired multilayer stackis between 0 and 5 mOhm, 0.25 and 3 mOhm, 0.3 and 1 mOhm, or any rangesubsumed therein. The sheet resistance of a metal particle layer isdetermined by the slope of the line times the length of the electrodes.The contact resistance and sheet resistance are used to numericallydetermine the transfer length and subsequently the contact resistivity.The change in series resistance is determined by dividing the contactresistivity by the fractional area coverage of the modifiedintercalation layer. In various embodiments the change in seriesresistance is less than 0.200 Ω-cm², less than 0.100 Ω-cm², less than0.050 106 -cm², less than 0.010 Ω-cm², or less than 0.001 Ω-cm².

The contact resistance between a rear tabbing layer and an aluminumparticle layer can affect the series resistance and power conversionefficiency of a solar cell. Such contact resistances can be measured bytransmission line measurements. A transmission line plot of aconventional silver rear tabbing layer on silicon with 300 μm overlapwith an aluminum particle layer is shown in FIG. 30. A transmission lineplot of a modified intercalation layer, used as a rear tabbing layer, onan aluminum particle layer is shown in FIG. 31. The y-intercept value inFIG. 31 is 1.11 mOhm as compared with a y-intercept value of 0.88 inFIG. 30. The contact resistance between a rear tabbing (intercalation)layer and an aluminum particle layer is 0.56 mOhm. The contactresistance for a conventional rear tabbing architecture is 0.44 mOhm. Invarious embodiments, the contact resistance between a rear tabbing(intercalation) layer and an aluminum particle layer is between 0 and 5mOhm, between 0.25 and 3 mOhm, or between 0.3 and 1 mOhm, or any rangesubsumed therein. The sheet resistance of an aluminum layer isdetermined by the slope of the line times the length of the electrodeand is approximately 9 mOhm/square in FIGS. 30 and 31.

Although TLM is a preferred method to accurately extract the contactresistance of a fired multilayer stack (i.e., rear tabbing layer andaluminum particle layer), it is possible to determine the contactresistance on a complete solar cell by using a four-point probe method.The method is used by first measuring the resistance between two reartabbing layers (R_(Ag-to-Ag)) and subsequently by moving the probes onto the Al particle layer (within 1 mm of the rear tabbing layer) to getR_(Al-to-Al). The contact resistance is determined by subtractingR_(Ag-to-Ag) from R_(Al-to-Al) and dividing by two. This is not asaccurate as TLM measurements but it can be approximate to within 0.50mOhm when averaging over measurements from multiple solar cells.

Contact resistance and sheet resistance are used to determinenumerically the transfer length and subsequently the contactresistivity. In FIG. 31, the transfer length of a co-fired multilayerstack is 5 mm and the contact resistance is 2.2 mΩ. The change in seriesresistance can be estimated by dividing this number by the fractionalarea coverage of the intercalation layer. In FIG. 31, the estimatedchange in series resistance is 0.023 Ω-cm², which is equivalent to anestimated change in series resistance of 0.020 Ω-cm² calculated for theconventional rear tabbing layer that was measured in FIG. 30. The changein series resistance can be directly measured by fabricating a controlBSF (back surface field) silicon solar cell with full Al coverage and norear tabbing layer and fabricating a BSF silicon solar cell with full Alcoverage and the Ag:Bi intercalation layer. The series resistance of thecells can be derived through the current-voltage curves under variouslight intensities and the difference in series resistances can beascribed to the increased contact resistance between the rear tabbingand the fired aluminum particle layer. In various embodiments, thechange in series resistance in the solar cell is less than 0.200 Ω-cm²,less than 0.100 Ω-cm², less than 0.050 Ω-cm², less than 0.010 Ω-cm², orless than 0.001 Ω-cm².

One benefit of using an intercalation layer on a silicon solar cell isthe improvement in open-circuit voltage (V_(oc)) caused by continuousback surface field formation in the silicon wafer. The V_(oc) gain canbe directly measured by comparing a conventional BSF solar cell to theBSF solar cell containing the Ag:Bi intercalation paste, as describedherein, when both devices have the same rear busbar surface area. Theconventional BSF silicon solar cell is fabricated with the silver basedrear tabbing paste printed directly onto a silicon wafer and surroundedby the aluminum particle layer. The intercalation layer (e.g.,fabricated using intercalation paste C) can be used on silicon solarcells that have full Al surface coverage. The V_(oc) of both solar cellsare measured through the current-voltage testing under one sun lightintensity. For solar cells that have more than 5 cm² of rear tabbingsurface area, the V_(oc) may be increased by at least 0.5 mV, by atleast 1mV, by at least 2 mV, or by at least 4 mV when using anintercalation layer versus a conventional, rear tabbing layer on siliconarchitecture. Finally, the short-circuit current density (J_(sc)) andthe fill factor may also be improved when using the intercalation layerarchitecture instead of the conventional rear tabbing design. Silverdoes not make ohmic contact to p-type silicon. A silver tabbing layerdirectly on p-type silicon reduces current collection which canestimated by performing electroluminescence or photoluminescencemeasurements on complete or incomplete solar cells. The increase inJ_(sc) can also be measured by testing cells with the intercalationarchitecture versus the rear tabbing layer directly on silicon. Anotherbenefit is an increase in the fill factor, which can be positivelychanged due to an increase in the V_(oc), a reduction in contactresistance, and/or changes in the recombination dynamics on the rearside of the solar cell.

It is to be understood that the inventions described herein can becarried out by different equipment, materials and devices, and thatvarious modifications, both as to the equipment and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

The invention claimed is:
 1. A method of forming a fired multilayerstack, the method comprising the steps of: a) applying a wet metalparticle layer on at least a portion of a surface of a substrate; b)drying the wet metal particle layer to form a dried metal particlelayer; c) applying a wet intercalation layer directly on at least aportion of the dried metal particle layer to form a multilayer stack;wherein the wet intercalation layer comprises: between 10 wt % and 70 wt% precious metal particles; at least 10 wt % intercalating particles;and organic vehicle; wherein the intercalating particles comprise one ormore selected from the group consisting of low temperature base metalparticles, crystalline metal oxide particles, and glass frit particles;d) drying the multilayer stack; and e) co-firing the multilayer stack toform the fired multilayer stack.
 2. The method of claim 1, wherein thewet metal particle layer comprises metal particles comprising a materialselected from the group comprising aluminum, copper, iron, nickel,molybdenum, tungsten, tantalum, titanium, steel, and alloys, composites,and other combinations thereof.
 3. The method of claim 1 furthercomprising, before step a), the step of depositing at least onedielectric layer onto at least a portion of the surface of the substrateand wherein step a) comprises applying the wet metal particle layerdirectly on at least a portion the dielectric layer.
 4. The method ofclaim 1, wherein each applying step comprises a method selectedindependently from the group consisting of screen printing, gravureprinting, spray deposition, slot coating, 3D printing and inkjetprinting.
 5. The method of claim 1, wherein step a) comprises screenprinting through a patterned screen to produce a wet metal particlelayer that has variable thickness.
 6. The method of claim 1, whereinsteps b) and d) comprise drying at a temperature below 500° C. for atime period between 1 second and 90 minutes.
 7. The method of claim 1,wherein step e) comprises rapidly heating to a temperature greater than600° C. for a time period between 0.5 second and 60 minutes in air. 8.The method of claim 1 further comprising step f) soldering a tabbingribbon onto a portion of the fired multilayer stack.
 9. The method ofclaim 1, wherein the low temperature base metal particles comprise amaterial selected from the group consisting of bismuth, tin, tellurium,antimony, lead, and alloys, composites, and other combinations thereof.10. The method of claim 1, wherein the crystalline metal oxide particlescomprise oxygen and a metal selected from the group consisting ofbismuth, tin, tellurium, antimony, lead, vanadium, chromium, molybdenum,boron, manganese, cobalt, and alloys, composites and other combinationsthereof.
 11. The method of claim 1, wherein the glass frit particlescomprises a material selected from a group consisting of antimony,arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium,chromium, cobalt, fluorine, gallium, germanium, hafnium, indium, iodine,iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum,niobium, potassium, rhenium, selenium, silicon, sodium, strontium,tellurium, tin, vanadium, zinc, zirconium, alloys thereof, oxidesthereof, composites thereof, and other combinations thereof.
 12. Amethod of forming a fired multilayer stack, the method comprising thesteps of: a) applying a wet metal particle layer on at least a portionof a surface of a substrate; b) drying the wet metal particle layer toform a dried metal particle layer; c) firing the dried metal particlelayer to form a metal particle layer; d) applying a wet intercalationlayer directly on at least a portion of the metal particle layer to forma multilayer stack; wherein the wet intercalation layer comprises:between 10 wt % and 70 wt % precious metal particles; at least 10 wt %intercalating particles ; and organic vehicle; wherein the intercalatingparticles comprise one or more selected from the group consisting of lowtemperature base metal particles, crystalline metal oxide particles, andglass frit particles; e) drying the multilayer stack; and f) firing themultilayer stack to form the fired multilayer stack.
 13. A method forfabricating a solar cell, the method comprising the steps of: a)providing a silicon wafer that has a front surface and a back surface;b) applying a wet aluminum particle layer on at least a portion of theback surface of the silicon wafer; c) drying the wet aluminum particlelayer to form an aluminum particle layer; d) applying a wetintercalation layer directly on at least a portion of the aluminumparticle layer to form a multilayer stack; wherein the wet intercalationlayer comprises: between 10 wt % and 70 wt % precious metal particles;at least 10 wt % intercalating particles comprising ; and organicvehicle; wherein the intercalating particles comprise one or moreselected from the group consisting of low temperature base metalparticles, crystalline metal oxide particles, and glass frit particles;e) drying the multilayer stack; f) applying a plurality of fine gridlines and at least one front busbar layer onto the front surface of thesilicon wafer; g) drying the plurality of fine grid lines and the atleast one front busbar layer to form a structure; and h) co-firing thestructure to form a silicon solar cell.
 14. The method of claim 13,further comprising between step a) and step b) the step of depositing atleast one dielectric layer onto a at least a portion of the back surfaceof the silicon wafer and wherein step b) comprises applying the wetaluminum particle layer directly on the dielectric layer.
 15. The methodof claim 13, wherein each applying step comprises a method selected fromthe group consisting of screen printing, gravure printing, spraydeposition, slot coating, 3D printing and inkjet printing.
 16. Themethod of claim 13, wherein step b) comprises screen printing through apatterned screen to produce a wet metal particle layer that has variablethickness.
 17. The method of claim 13 wherein steps e) and g) comprisedrying at a temperature between 150° C. and 300° C. for a time periodbetween 1 second and 60 minutes.
 18. 13The method of claim 13 whereinco-firing comprises rapidly heating to a temperature greater than 700°C. for a time period between 0.5 and 3 seconds in air.
 19. The method ofclaim 13, wherein the low temperature base metal particles comprise amaterial selected from the group consisting of bismuth, tin, tellurium,antimony, lead, and alloys, composites, and other combinations thereof.20. The method of claim 13, wherein the crystalline metal oxideparticles comprise oxygen and a metal selected from the group consistingof bismuth, tin, tellurium, antimony, lead, vanadium, chromium,molybdenum, boron, manganese, cobalt, and alloys, composites and othercombinations thereof.
 21. The method of claim 13, wherein the glass fritparticles comprises a material selected from a group consisting ofantimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium,cesium, chromium, cobalt, fluorine, gallium, germanium, hafnium, indium,iodine, iron, lanthanum, lead, lithium, magnesium, manganese,molybdenum, niobium, potassium, rhenium, selenium, silicon, sodium,strontium, tellurium, tin, vanadium, zinc, zirconium, alloys thereof,oxides thereof, composites thereof, and other combinations thereof.